build: Add feature test flags to project-wide preprocessor flags

Rather than defining them piecemeal in each C and H file, and having
problems with headers being included in different orders so that
`_POSIX_C_SOURCE` gets a value which is too low, just define the feature
test flags in `meson.build`.

See `man 7 feature_test_macros`.

This is useful for when building sysprof as a subproject beneath a
project which doesn’t define these macros, or which targets an older C
standard than `gnu11`.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

meson.build

@@ -87,7 +87,10 @@ add_project_arguments([
   '-I' + meson.build_root(), # config.h
 ], language: 'c')
 
-global_c_args = []
+global_c_args = [
+  '-D_GNU_SOURCE',
+  '-D_POSIX_C_SOURCE=200809L',
+]
 test_c_args = [
   '-Wcast-align',
   '-Wdeclaration-after-statement',