Merge branch 'master' into core-updates

Remove now unneeded (guix build gnome) module.

* guix/build/gnome.scm: Remove.
* Makefile.am (MODULES): Update accordingly.

gnu: gtk+: enable introspection.

* gnu/packages/gtk.scm (atk, gdk-pixbuf, gtk+, pango): enable introspection
* guix/build/gnome.scm: New file.