default GTK_MODULES search dir & pkg-config

[Tim Janik <timj gtk org>]

hi owen, havoc,

we still need a common directory that gtk+ modules can install
themselves into. some ${prefix}/gtk+-2.0/modules directory or
so that gtk_main() will try to load modules from.
two issue with this:
1) you have the exact dir structure of gtk+ in mind, think
   any other place would be better?
2) when compiling against a certain gtk+ version, modules need to
   figure where to install themselves to. gtk-config is gone, so
   how do we aproach that with pgk-config?

for (2) we could have (in .pc):
Variables: moduledir="${prefix}/gtk+-2.0/modules"

and support
pkg-config --modvar=moduledir gtk+-2.0

unless you'd want to support "moduledir" directly in pkg-config.

---
ciaoTJ