meson: Set _GNU_SOURCE

When we start adding non-free software platforms support we'll need to
guard this, but for now it should be fine as is.

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

meson.build

@@ -31,6 +31,7 @@ pre_args = [
   '-DVERSION="@0@"'.format(meson.project_version()),
   '-DPACKAGE_VERSION=VERSION',
   '-DPACKAGE_BUGREPORT="https://bugs.freedesktop.org/enter_bug.cgi?product=Mesa"',
+  '-D_GNU_SOURCE',
 ]
 
 with_vulkan_icd_dir = get_option('vulkan-icd-dir')