meson: Define _GNU_SOURCE as a project argument
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

johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib