Normally linking with wxWidgets should be enough, even for wxGTK.
But if you have to use functions that aren't available in the API,
then use $(WX_GTK2_LIBS) and $(WX_X11_LIBS)* rather than
`pkg-config gtk+-2.0 --libs` and -lX11 directly, since otherwise
it will break on other wxWidgets platforms (wxMSW and wxMac).
* Where variable "X11_LIBS" is like a shorthand for "$X_LIBS -lX11".
It's not enough to check for GTK2/X presence, since they might be
there but not used - like when compiling on Mac OS X (it has X11).
So the variables prefixed with WX_ are used like "when on wxGTK",
on other wxWidgets platform they will be empty even with GTK2/X.
