Erlang wxgtk

Add --with-wxgtk to use wxGTK and be able to use erlang's graphic tools like observer in Mountain Lion.

Erlang depends on wxWidgets 2.8 API for GUI tools like observer. In Mountain Lion WxWidgets 2.8 do not compile because Carbon has been dropped from XCode 4.4.
To solve this issue WxGTK can be used as it has the same API as WxWidgets 2.8 and use X11 instead of Carbon.