build: Add architecture checks for x86 and ARM

On x86, SSE optimisations are always compiled in, and used based on
runtime checks.

On ARM, we try to autodetect NEON support (with an option of runtime
detection). This has not been build-tested on ARM yet.

This leaves MIPS to be done.

configure.ac

@@ -7,10 +7,14 @@ AC_SUBST(LIBWEBRTC_AUDIO_PROCESSING_VERSION_INFO, [0:0:0])
 
 AM_SILENT_RULES([yes])
 
+# Set up the host_* variables
+AC_CANONICAL_HOST
+
 AC_PROG_CC
 AC_PROG_CXX
 AC_PROG_LIBTOOL
 AC_PROG_INSTALL
+AM_PROG_AS
 
 AC_LANG_C
 AC_LANG_CPLUSPLUS
@@ -53,8 +57,51 @@ AS_CASE(["${host}"],
 )
 AC_SUBST(PLATFORM_CFLAGS)
 
-COMMON_CFLAGS="-DWEBRTC_AUDIO_PROCESSING_ONLY_BUILD ${PLATFORM_CFLAGS} ${OS_CFLAGS} -DNDEBUG -I\$(top_srcdir)"
-COMMON_CXXFLAGS="-std=c++11 -DWEBRTC_AUDIO_PROCESSING_ONLY_BUILD ${PLATFORM_CFLAGS} ${OS_CFLAGS} -DNDEBUG -I\$(top_srcdir)"
+AS_CASE(["${host_cpu}"],
+    [i?86|x86_64],
+        [
+         HAVE_X86=1
+        ],
+    [armv7*|armv8*],
+        [
+         HAVE_ARM=1
+         HAVE_ARMV7=1
+        ],
+    [arm*],
+        [
+         HAVE_ARM=1
+        ]
+    # FIXME: Add MIPS support, see webrtc/BUILD.gn for defines
+)
+AM_CONDITIONAL(HAVE_X86, [test "x${HAVE_X86}" = "x1"])
+AM_CONDITIONAL(HAVE_ARM, [test "x${HAVE_ARM}" = "x1"])
+AM_CONDITIONAL(HAVE_ARMV7, [test "x${HAVE_ARMV7}" = "x1"])
+
+# Borrowed from pulseaudio's configure.ac
+AC_ARG_ENABLE([neon],
+    AS_HELP_STRING([--enable-neon], [Enable NEON optimisations on ARM CPUs that support it (yes|no|auto|runtime)]))
+
+AS_IF([test "x$enable_neon" != "xno"],
+    AS_IF([test "x$enable_neon" != "xruntime"],
+        [
+         save_CFLAGS="$CFLAGS"; CFLAGS="-mfpu=neon $CFLAGS"
+         AC_COMPILE_IFELSE(
+            [AC_LANG_PROGRAM([[#include <arm_neon.h>]], [])],
+            [
+             HAVE_NEON=1
+             ARCH_CFLAGS="-mfpu=neon"
+            ])
+         CFLAGS="$save_CFLAGS"
+        ],
+        [
+         HAVE_NEON=1
+         ARCH_CFLAGS="-mfpu=neon -DWEBRTC_DETECT_NEON"
+        ])
+)
+AM_CONDITIONAL([HAVE_NEON], [test "x$HAVE_NEON" = "x1"])
+
+COMMON_CFLAGS="-DWEBRTC_AUDIO_PROCESSING_ONLY_BUILD ${PLATFORM_CFLAGS} ${OS_CFLAGS} ${ARCH_CFLAGS} -DNDEBUG -I\$(top_srcdir)"
+COMMON_CXXFLAGS="-std=c++11 -DWEBRTC_AUDIO_PROCESSING_ONLY_BUILD ${PLATFORM_CFLAGS} ${OS_CFLAGS} ${ARCH_CFLAGS} -DNDEBUG -I\$(top_srcdir)"
 AC_SUBST([COMMON_CFLAGS])
 AC_SUBST([COMMON_CXXFLAGS])