[poppler] Build compile if gtk2-devel is not installed

[Albert Astals Cid]

If gtk2-devel is not installed i get

checking for gtk+-2.0 >= 2.2.0... Package gtk+-2.0 was not found in the 
pkg-config search path.
Perhaps you should add the directory containing `gtk+-2.0.pc'
to the PKG_CONFIG_PATH environment variable
No package 'gtk+-2.0' found

I guess something like the cairo fix Brad sent could be used to disable 
building the gtk-2 test binary if gtk-2 is not found instead of dyieng.

Albert (who does not has gtk-2 devel in one of his machines)