We're not going to depend on gnome-common (I assume) so this patch
nicks the systemd macro to test for compiler flags, and uses it to set
a similar set of -Werror=foo as the gnome-common one does.
See https://bugzilla.gnome.org/show_bug.cgi?id=608953
See https://mail.gnome.org/archives/desktop-devel-list/2012-July/msg00100.html
If we're going to be setting more strict compiler flags for GNOME, we
should really ensure GLib builds with them first, as it's kind of the
model citizen.
In particular, you can see several times that downstreams such as
Debian have come in and fixed -Wformat-security bugs. We should never
let those get into tarballs, or even commits.
https://bugzilla.gnome.org/show_bug.cgi?id=687385
