Provide a fallback luaL_setfuncs() for Lua 5.1

The LGI bindings are failing at runtime because of the above missing
function. Added it to compat.c: implementation borrowed from Lua wiki.
Fix compatibility with Gtk+ 2.x: Gdk.RGBA is only available in Gtk+ 3.
Merge pull request #52 from Trundle/master

Small Gtk+ 2 fix: Gdk.RGBA is new in Gtk+ 3
Merge branch 'pr3' into devel

Supersede branch pr3, not needed. Check pull request #51 for details:
http://github.com/pavouk/lgi/pull/51
Merge branch 'upstream' into devel