diff --git a/compilers/Clang.C.cmake b/compilers/Clang.C.cmake
index ef1483e..8f3f3af 100644
--- a/compilers/Clang.C.cmake
+++ b/compilers/Clang.C.cmake
@@ -1,5 +1,7 @@
-if(CMAKE_C_COMPILER_ID MATCHES Clang)
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
-    set(CMAKE_C_FLAGS_RELEASE "-O3 -Wno-unused")
-    set(CMAKE_C_FLAGS_DEBUG "-O0 -g")
+if(NOT DEFINED ENV{CFLAGS})
+    if(CMAKE_C_COMPILER_ID MATCHES Clang)
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
+        set(CMAKE_C_FLAGS_RELEASE "-O3")
+        set(CMAKE_C_FLAGS_DEBUG "-O0 -g")
+    endif()
 endif()
diff --git a/compilers/Clang.CXX.cmake b/compilers/Clang.CXX.cmake
index 0d0998a..5f13571 100644
--- a/compilers/Clang.CXX.cmake
+++ b/compilers/Clang.CXX.cmake
@@ -1,5 +1,7 @@
-if(CMAKE_CXX_COMPILER_ID MATCHES Clang)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wno-unknown-pragmas -Wno-sign-compare -Woverloaded-virtual -Wwrite-strings -Wno-unused")
-    set(CMAKE_CXX_FLAGS_RELEASE "-Ofast -march=native -DNDEBUG")
-    set(CMAKE_CXX_FLAGS_DEBUG "-O0 -DDEBUG -g")
+if(NOT DEFINED ENV{CXXFLAGS})
+    if(CMAKE_CXX_COMPILER_ID MATCHES Clang)
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
+        set(CMAKE_CXX_FLAGS_RELEASE "-Ofast")
+        set(CMAKE_CXX_FLAGS_DEBUG "-O0 -g")
+    endif()
 endif()
diff --git a/compilers/GNU.C.cmake b/compilers/GNU.C.cmake
index ce8384c..282d243 100644
--- a/compilers/GNU.C.cmake
+++ b/compilers/GNU.C.cmake
@@ -1,5 +1,7 @@
-if(CMAKE_C_COMPILER_ID MATCHES GNU)
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
-    set(CMAKE_C_FLAGS_RELEASE "-O3 -Wno-unused")
-    set(CMAKE_C_FLAGS_DEBUG "-O0 -g")
+if(NOT DEFINED ENV{CFLAGS})
+    if(CMAKE_C_COMPILER_ID MATCHES GNU)
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
+        set(CMAKE_C_FLAGS_RELEASE "-O3")
+        set(CMAKE_C_FLAGS_DEBUG "-O0 -g")
+    endif()
 endif()
diff --git a/compilers/GNU.CXX.cmake b/compilers/GNU.CXX.cmake
index 768abd1..6d0ad14 100644
--- a/compilers/GNU.CXX.cmake
+++ b/compilers/GNU.CXX.cmake
@@ -1,5 +1,7 @@
-if(CMAKE_CXX_COMPILER_ID MATCHES GNU)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wno-unknown-pragmas -Wno-sign-compare -Woverloaded-virtual -Wwrite-strings -Wno-unused")
-    set(CMAKE_CXX_FLAGS_RELEASE "-Ofast -march=native -DNDEBUG")
-    set(CMAKE_CXX_FLAGS_DEBUG "-O0 -DDEBUG -g")
+if(NOT DEFINED ENV{CXXFLAGS})
+    if(CMAKE_CXX_COMPILER_ID MATCHES GNU)
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
+        set(CMAKE_CXX_FLAGS_RELEASE "-Ofast")
+        set(CMAKE_CXX_FLAGS_DEBUG "-O0 -g")
+    endif()
 endif()
diff --git a/compilers/GNU.Fortran.cmake b/compilers/GNU.Fortran.cmake
index cab129e..99a8a69 100644
--- a/compilers/GNU.Fortran.cmake
+++ b/compilers/GNU.Fortran.cmake
@@ -1,7 +1,7 @@
 if(NOT DEFINED ENV{FCFLAGS})
     if(CMAKE_Fortran_COMPILER_ID MATCHES GNU)
         set(CMAKE_Fortran_FLAGS "${CMAKE_Fortran_FLAGS}")
-        set(CMAKE_Fortran_FLAGS_RELEASE "-O3 -funroll-all-loops -w")
+        set(CMAKE_Fortran_FLAGS_RELEASE "-O3 -funroll-all-loops")
         set(CMAKE_Fortran_FLAGS_DEBUG "-O0 -g -fbacktrace")
     endif()
 endif()
diff --git a/compilers/Intel.C.cmake b/compilers/Intel.C.cmake
index b2c1847..1f2a529 100644
--- a/compilers/Intel.C.cmake
+++ b/compilers/Intel.C.cmake
@@ -1,5 +1,7 @@
-if(CMAKE_C_COMPILER_ID MATCHES Intel)
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
-    set(CMAKE_C_FLAGS_RELEASE "-O3")
-    set(CMAKE_C_FLAGS_DEBUG "-O0 -g")
+if(NOT DEFINED ENV{CFLAGS})
+    if(CMAKE_C_COMPILER_ID MATCHES Intel)
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
+        set(CMAKE_C_FLAGS_RELEASE "-O3")
+        set(CMAKE_C_FLAGS_DEBUG "-O0 -g")
+    endif()
 endif()
diff --git a/compilers/Intel.CXX.cmake b/compilers/Intel.CXX.cmake
index 363ff39..e4700b5 100644
--- a/compilers/Intel.CXX.cmake
+++ b/compilers/Intel.CXX.cmake
@@ -1,5 +1,7 @@
-if(CMAKE_CXX_COMPILER_ID MATCHES Intel)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-unknown-pragmas")
-    set(CMAKE_CXX_FLAGS_RELEASE "-O3 -DNDEBUG")
-    set(CMAKE_CXX_FLAGS_DEBUG "-O0 -debug -DDEBUG")
+if(NOT DEFINED ENV{CXXFLAGS})
+    if(CMAKE_CXX_COMPILER_ID MATCHES Intel)
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
+        set(CMAKE_CXX_FLAGS_RELEASE "-O3")
+        set(CMAKE_CXX_FLAGS_DEBUG "-O0 -g")
+    endif()
 endif()
diff --git a/compilers/Intel.Fortran.cmake b/compilers/Intel.Fortran.cmake
index 6b4ce35..7b7c57c 100644
--- a/compilers/Intel.Fortran.cmake
+++ b/compilers/Intel.Fortran.cmake
@@ -1,6 +1,6 @@
 if(NOT DEFINED ENV{FCFLAGS})
     if(CMAKE_Fortran_COMPILER_ID MATCHES Intel)
-        set(CMAKE_Fortran_FLAGS "${CMAKE_Fortran_FLAGS} -w -assume byterecl")
+        set(CMAKE_Fortran_FLAGS "${CMAKE_Fortran_FLAGS}")
         set(CMAKE_Fortran_FLAGS_RELEASE "-O3 -ip")
         set(CMAKE_Fortran_FLAGS_DEBUG "-O0 -g -traceback")
     endif()
diff --git a/compilers/PGI.C.cmake b/compilers/PGI.C.cmake
index 8f7e460..27f82b3 100644
--- a/compilers/PGI.C.cmake
+++ b/compilers/PGI.C.cmake
@@ -1,5 +1,7 @@
-if(CMAKE_C_COMPILER_ID MATCHES PGI)
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
-    set(CMAKE_C_FLAGS_RELEASE "-O3")
-    set(CMAKE_C_FLAGS_DEBUG "-O0 -g")
+if(NOT DEFINED ENV{CFLAGS})
+    if(CMAKE_C_COMPILER_ID MATCHES PGI)
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
+        set(CMAKE_C_FLAGS_RELEASE "-O3")
+        set(CMAKE_C_FLAGS_DEBUG "-O0 -g")
+    endif()
 endif()
diff --git a/compilers/PGI.CXX.cmake b/compilers/PGI.CXX.cmake
index 09815be..bd62763 100644
--- a/compilers/PGI.CXX.cmake
+++ b/compilers/PGI.CXX.cmake
@@ -1,5 +1,7 @@
-if(CMAKE_CXX_COMPILER_ID MATCHES PGI)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
-    set(CMAKE_CXX_FLAGS_RELEASE "-O3")
-    set(CMAKE_CXX_FLAGS_DEBUG "-O0 -g")
+if(NOT DEFINED ENV{CXXFLAGS})
+    if(CMAKE_CXX_COMPILER_ID MATCHES PGI)
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
+        set(CMAKE_CXX_FLAGS_RELEASE "-O3")
+        set(CMAKE_CXX_FLAGS_DEBUG "-O0 -g")
+    endif()
 endif()
diff --git a/compilers/XL.C.cmake b/compilers/XL.C.cmake
index 5afd485..f70cf62 100644
--- a/compilers/XL.C.cmake
+++ b/compilers/XL.C.cmake
@@ -1,5 +1,7 @@
-if(CMAKE_C_COMPILER_ID MATCHES XL)
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -qcpluscmt")
-    set(CMAKE_C_FLAGS_RELEASE "-O3")
-    set(CMAKE_C_FLAGS_DEBUG "-O0")
+if(NOT DEFINED ENV{CFLAGS})
+    if(CMAKE_C_COMPILER_ID MATCHES XL)
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -qcpluscmt")
+        set(CMAKE_C_FLAGS_RELEASE "-O3")
+        set(CMAKE_C_FLAGS_DEBUG "-O0 -g")
+    endif()
 endif()
diff --git a/compilers/XL.CXX.cmake b/compilers/XL.CXX.cmake
index 69f974f..ff73459 100644
--- a/compilers/XL.CXX.cmake
+++ b/compilers/XL.CXX.cmake
@@ -1,5 +1,7 @@
-if(CMAKE_CXX_COMPILER_ID MATCHES XL)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
-    set(CMAKE_CXX_FLAGS_RELEASE "-O3")
-    set(CMAKE_CXX_FLAGS_DEBUG "-O0 -g")
+if(NOT DEFINED ENV{CXXFLAGS})
+    if(CMAKE_CXX_COMPILER_ID MATCHES XL)
+        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
+        set(CMAKE_CXX_FLAGS_RELEASE "-O3")
+        set(CMAKE_CXX_FLAGS_DEBUG "-O0 -g")
+    endif()
 endif()
diff --git a/compilers/XL.Fortran.cmake b/compilers/XL.Fortran.cmake
index 23f9a4b..637fa5f 100644
--- a/compilers/XL.Fortran.cmake
+++ b/compilers/XL.Fortran.cmake
@@ -1,6 +1,6 @@
 if(NOT DEFINED ENV{FCFLAGS})
     if(CMAKE_Fortran_COMPILER_ID MATCHES XL)
-        set(CMAKE_Fortran_FLAGS "${CMAKE_Fortran_FLAGS} -qzerosize -qextname -qsuppress=cmpmsg")
+        set(CMAKE_Fortran_FLAGS "${CMAKE_Fortran_FLAGS}")
         set(CMAKE_Fortran_FLAGS_RELEASE "-O3")
         set(CMAKE_Fortran_FLAGS_DEBUG "-g")
     endif()
diff --git a/modules/cc.cmake b/modules/cc.cmake
index a4affc6..e7ae618 100644
--- a/modules/cc.cmake
+++ b/modules/cc.cmake
@@ -1,3 +1,31 @@
+#.rst:
+#
+# Adds C support.
+# Appends EXTRA_CFLAGS to CMAKE_C_FLAGS.
+# If environment variable CFLAGS is set, then the CFLAGS are used
+# and no other flags are used or appended.
+#
+# Variables used::
+#
+#   EXTRA_CFLAGS
+#
+# Variables modified::
+#
+#   CMAKE_C_FLAGS
+#
+# Environment variables used::
+#
+#   CFLAGS
+#
+# Example autocmake.cfg entry::
+#
+#   [cc]
+#   source: https://github.com/scisoft/autocmake/raw/master/modules/cc.cmake
+#   docopt: --cc=<CC> C compiler [default: gcc].
+#           --extra-cc-flags=<EXTRA_CFLAGS> Extra C compiler flags [default: ''].
+#   export: 'CC=%s' % arguments['--cc']
+#   define: '-DEXTRA_CFLAGS="%s"' % arguments['--extra-cc-flags']
+
 enable_language(C)
 
 if(NOT DEFINED CMAKE_C_COMPILER_ID)
@@ -8,6 +36,11 @@ if(NOT CMAKE_C_COMPILER_WORKS)
     message(FATAL_ERROR "CMAKE_C_COMPILER_WORKS is false!")
 endif()
 
-if(DEFINED EXTRA_C_FLAGS)
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${EXTRA_C_FLAGS}")
+if(DEFINED EXTRA_CFLAGS)
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${EXTRA_CFLAGS}")
+endif()
+
+if(DEFINED ENV{CFLAGS})
+    message(STATUS "CFLAGS is set to '$ENV{CFLAGS}'.")
+    set(CMAKE_C_FLAGS "$ENV{CFLAGS}")
 endif()
diff --git a/modules/cxx.cmake b/modules/cxx.cmake
index 086c27a..21cc4ca 100644
--- a/modules/cxx.cmake
+++ b/modules/cxx.cmake
@@ -1,13 +1,46 @@
+#.rst:
+#
+# Adds C++ support.
+# Appends EXTRA_CXXFLAGS to CMAKE_CXX_FLAGS.
+# If environment variable CXXFLAGS is set, then the CXXFLAGS are used
+# and no other flags are used or appended.
+#
+# Variables used::
+#
+#   EXTRA_CXXFLAGS
+#
+# Variables modified::
+#
+#   CMAKE_CXX_FLAGS
+#
+# Environment variables used::
+#
+#   CXXFLAGS
+#
+# Example autocmake.cfg entry::
+#
+#   [cxx]
+#   source: https://github.com/scisoft/autocmake/raw/master/modules/cxx.cmake
+#   docopt: --cxx=<CXX> C++ compiler [default: g++].
+#           --extra-cxx-flags=<EXTRA_CXXFLAGS> Extra C++ compiler flags [default: ''].
+#   export: 'CXX=%s' % arguments['--cxx']
+#   define: '-DEXTRA_CXXFLAGS="%s"' % arguments['--extra-cxx-flags']
+
 enable_language(CXX)
 
-if(NOT DEFINED CMAKE_CXX_COMPILER_ID)
-    message(FATAL_ERROR "CMAKE_CXX_COMPILER_ID variable is not defined!")
+if(NOT DEFINED CMAKE_C_COMPILER_ID)
+    message(FATAL_ERROR "CMAKE_C_COMPILER_ID variable is not defined!")
 endif()
 
-if(NOT CMAKE_CXX_COMPILER_WORKS)
-    message(FATAL_ERROR "CMAKE_CXX_COMPILER_WORKS is false!")
+if(NOT CMAKE_C_COMPILER_WORKS)
+    message(FATAL_ERROR "CMAKE_C_COMPILER_WORKS is false!")
 endif()
 
-if(DEFINED EXTRA_CXX_FLAGS)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${EXTRA_CXX_FLAGS}")
+if(DEFINED EXTRA_CXXFLAGS)
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${EXTRA_CXXFLAGS}")
+endif()
+
+if(DEFINED ENV{CXXFLAGS})
+    message(STATUS "CXXFLAGS is set to '$ENV{CXXFLAGS}'.")
+    set(CMAKE_CXX_FLAGS "$ENV{CXXFLAGS}")
 endif()
diff --git a/modules/fc.cmake b/modules/fc.cmake
index a9048b4..c4b005f 100644
--- a/modules/fc.cmake
+++ b/modules/fc.cmake
@@ -5,10 +5,6 @@
 # If environment variable FCFLAGS is set, then the FCFLAGS are used
 # and no other flags are used or appended.
 #
-# Environment variables used::
-#
-#   FCFLAGS
-#
 # Variables used::
 #
 #   EXTRA_FCFLAGS
@@ -21,6 +17,10 @@
 #
 #   CMAKE_Fortran_FLAGS
 #
+# Environment variables used::
+#
+#   FCFLAGS
+#
 # Example autocmake.cfg entry::
 #
 #   [fc]
diff --git a/test/cxx/cmake/autocmake.cfg b/test/cxx/cmake/autocmake.cfg
index 876cdc3..0d0494b 100644
--- a/test/cxx/cmake/autocmake.cfg
+++ b/test/cxx/cmake/autocmake.cfg
@@ -4,7 +4,9 @@ name: example
 [cxx]
 source: https://github.com/scisoft/autocmake/raw/master/modules/cxx.cmake
 docopt: --cxx=<CXX> C++ compiler [default: g++].
+        --extra-cxx-flags=<EXTRA_CXXFLAGS> Extra C++ compiler flags [default: ''].
 export: 'CXX=%s' % arguments['--cxx']
+define: '-DEXTRA_CXXFLAGS="%s"' % arguments['--extra-cxx-flags']
 
 [default_build_paths]
 source: https://github.com/scisoft/autocmake/raw/master/modules/default_build_paths.cmake