gnome-common updated with some explicit -Werror flags

[Colin Walters <walters verbum org>]

Users of gnome-common, please take a look at:

https://bugzilla.gnome.org/show_bug.cgi?id=608953

I checked everything in the current gnome-ostree 3.8 manifest builds,
but there is likely more out there.  If you hit anything, please comment
on the bug and we can work out whether we should drop anything from the
base set of -Werror=foo flags.

Previous discussion:
https://mail.gnome.org/archives/desktop-devel-list/2012-July/msg00100.html