gmacros.h: add G_GNUC_*_IGNORE_DEPRECATIONS macros for clang

https://bugzilla.gnome.org/show_bug.cgi?id=734126

Signed-off-by: Thomas Haller <thaller@redhat.com>
(cherry picked from commit 35eaf037bdfca985abf5d349e7355f1d2ed9c77b)