Add -D*_DISABLE_SINGLE_INCLUDES to GNOME_MAINTAINER_MODE_DEFINES

[Cosimo Cecchi <cosimoc gnome org>]

Hi everyone,

currently we have the GNOME_MAINTAINER_MODE_DEFINES macro in
gnome-common, which takes care of enabling the -D*_DISABLE_DEPRECATED
switches for libraries in the stack.
Given the new GLib/GTK+ single header policy, it would make sense to
also add the *_DISABLE_SINGLE_INCLUDES switch to that macro, so that
every module could adapt to the new policy easily.
I filed a bug [1] against gnome-common, comments welcome.

[1] http://bugzilla.gnome.org/show_bug.cgi?id=557070

Cheers,

Cosimo