Introduce a config option for building on OS X GTK.

Introduce a config option for building on OS X GTK.

Using GTK to run on OS X is going to require several workarounds and
behaviour tweaks to be enabled at various points in the code, and it's
already getting cumbersome to remember what they all are to put on the
command line. Here's a central #define (OSX_GTK) that enables them all
in one go, and a configure option (--with-quartz) that sets it.

As part of this commit, I've also rearranged the #include order in the
GTK source files, so that they include unix.h (which now might be
where NOT_X_WINDOWS gets defined) before they test NOT_X_WINDOWS to
decide whether to include X11 headers.