[glib] meson: Define _GNU_SOURCE as a project argument

[Emmanuele Bassi <ebassi src gnome org>]

commit 1a755a63e1c632ae57de8117e9e03497c19d9b20
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Wed Aug 9 21:29:13 2017 +0100

    meson: Define _GNU_SOURCE as a project argument
    
    We use it pretty much everywhere in order to get feature detection, and
    that's also what the AC_USE_SYSTEM_EXTENSIONS m4 macro defines in the
    Autotools build.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=785955

 meson.build |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/meson.build b/meson.build
index 35baaaa..5a37cde 100644
--- a/meson.build
+++ b/meson.build
@@ -58,6 +58,8 @@ glib_giomodulesdir = join_paths(glib_libdir, 'gio', 'modules')
 
 glib_pkgconfigreldir = join_paths(glib_libdir, 'pkgconfig')
 
+add_project_arguments('-D_GNU_SOURCE', language: 'c')
+
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