Splitting pangomm out of gtkmm

[Murray Cumming <murrayc murrayc com>]

pangomm is currently in the gtkmm source tarball, though it is a
separate library (.so file, headers directory, .pc file, etc). We plan
to move it into a separate tarball, to make it easier for cluttermm to
depend on it without depending on gtkmm at build time.

I don't think that is an ABI or API break, and I think it would be
fairly easy for packagers to adjust, because sane distros already
package it separately. Please tell me if you have objections.
 
-- 
Murray Cumming
murrayc murrayc com
www.murrayc.com
www.openismus.com