meson.build: Force-include msvc_recommended_pragmas.h
meson.build: Force-include msvc_recommended_pragmas.h

...on Visual Studio builds.  We can get rid of lots of compiler noise
with that header, which we are free to use since we are always using
GLib.

Also, define _GNU_SOURCE when we are not using Visual Studio.

GNOME/clutter,GNOME/clutter,GNOME/clutter,GNOME/clutter,GNOME/clutter