[GtkGLExt] Build system fixes

[Braden McDaniel <braden endoframe com>]

I've got a number of build system fixes to accommodate non-srcdir builds
and at least one fix for Cygwin. Is there interest in these? If so, I'd
be happy to post them to the SourceForge patch manager. If not, I won't
bother.

-- 
Braden McDaniel                           e-mail: <braden endoframe com>
<http://endoframe.com>                    Jabber: <braden jabber org>