build: Opt-in to Darwin extensions

In particular, this might give us access to `st_mtim.tv_nsec`. We
already opt-in to GNU and QNX extensions on a similar basis.

Signed-off-by: Simon McVittie <smcv@collabora.com>

meson.build

@@ -130,6 +130,10 @@ if host_system == 'qnx'
   add_project_arguments('-D_QNX_SOURCE', language: 'c')
 endif
 
+if host_system == 'darwin'
+  add_project_arguments('-D_DARWIN_C_SOURCE', language: 'c')
+endif
+
 # Disable strict aliasing;
 # see https://bugzilla.gnome.org/show_bug.cgi?id=791622
 if cc.has_argument('-fno-strict-aliasing')