Disable deprecation checks for all libraries we depend on on stable branches.

Disable deprecation checks for all libraries we depend on on stable branches.

This is so newer versions of those libraries don't cause more warnings
with a stable GTK version.
We don't ever want to turn off deprecation warnings for master however,
because that's where we get rid of deprecated API we use.

Note that only glib allows use to easily do this, so nothing is done for
Pango, gdk-pixbuf or Cairo here.