meson: Get rid of cmake and manual library searching

Simplify fallback, and prefer it. `[provide]` section requires meson
0.55, so require that.

pkg-config lookup is only provided for distros, since they dislike
static linking / vendoring.

meson.build

@@ -1,6 +1,6 @@
 project('webrtc-audio-processing', 'c', 'cpp',
   version : '1.1',
-  meson_version : '>= 0.54',
+  meson_version : '>= 0.55',
   default_options : [ 'warning_level=1',
                       'buildtype=debugoptimized',
                       'c_std=c11',
@@ -41,84 +41,15 @@ have_posix = false
 have_win = false
 
 # Let's use pkg-config if available. This will also fallback to the subproject
-# if pkg-config is not found, instead of CMake or manual library detection.
-# This might be surprising, but is hopefully a temporary state until recent
-# abseil versions become the norm.
-absl_base_dep = dependency('absl_base', required : false,
-  fallback: [ 'abseil-cpp', 'absl_base_dep' ]
-)
-absl_flags_dep = dependency('absl_flags_parse', required : false,
-  fallback: [ 'abseil-cpp', 'absl_flags_dep' ]
-)
-absl_optional_dep = dependency('absl_optional', required : false,
-  fallback: [ 'abseil-cpp', 'absl_types_dep' ]
-)
-absl_strings_dep = dependency('absl_strings', required : false,
-  fallback: [ 'abseil-cpp', 'absl_strings_dep' ]
-)
-absl_synchronization_dep = dependency('absl_synchronization', required : false,
-  fallback: [ 'abseil-cpp', 'absl_synchronization_dep' ]
-)
-
-# If we have the base dep, assume the rest should be present to
-if absl_base_dep.found()
-  absl_dep = [
-    absl_base_dep,
-    absl_flags_dep,
-    absl_optional_dep,
-    absl_strings_dep,
-    absl_synchronization_dep,
-  ]
-else
-  warning('Could not find abseil-cpp with pkg-config, trying CMake-based library detection.')
-  absl_dep = dependency('absl', method : 'cmake',
-    modules : [
-      'absl::base',
-      'absl::flags_parse',
-      'absl::optional',
-      'absl::strings',
-      'absl::synchronization',
-    ],
-    required : false,
-  )
-
-  if not absl_dep.found()
-    warning('Could not find abseil-cpp with CMake, using fallback library detection which may fail.')
-    absl_libs = [
-      'absl_base',
-      'absl_bad_optional_access',
-      'absl_city',
-      'absl_flags_commandlineflag',
-      'absl_flags_commandlineflag_internal',
-      'absl_flags_config',
-      'absl_flags_internal',
-      'absl_flags_marshalling',
-      'absl_flags_parse',
-      'absl_flags_private_handle_accessor',
-      'absl_flags_program_name',
-      'absl_flags_reflection',
-      'absl_flags_usage',
-      'absl_flags_usage_internal',
-      'absl_hash',
-      'absl_int128',
-      'absl_malloc_internal',
-      'absl_raw_logging_internal',
-      'absl_spinlock_wait',
-      'absl_stacktrace',
-      'absl_str_format_internal',
-      'absl_strings',
-      'absl_symbolize',
-      'absl_synchronization',
-      'absl_throw_delegate',
-      'absl_time',
-      'absl_time_zone',
-    ]
-    absl_dep = []
-    foreach l : absl_libs
-      absl_dep += cpp.find_library(l, required : false)
-    endforeach
-  endif
-endif
+# if pkg-config is not found, which is really the most reliable way of building
+# abseil due to strict C++ standard match requirements.
+absl_dep = [
+  dependency('absl_base'),
+  dependency('absl_flags'),
+  dependency('absl_types'),
+  dependency('absl_strings'),
+  dependency('absl_synchronization'),
+]
 
 if ['darwin', 'ios'].contains(host_system)
   os_cflags = ['-DWEBRTC_MAC']