Because gtkglext uses pango_x_font_cache_load() in gdk/x11/gdkglfont-x11.c, it needs to link against libpangox, which it does not do. It also needs to specify the dependency in gdkglext.pc.
Created attachment 68115 [details, diff] gtkglext-1.0.6-requires-pangox.patch Patch to fix the issue. (needs running autoconf after applying)
Thanks Az, committed to CVS.
*** Bug 114873 has been marked as a duplicate of this bug. ***