Configure now generates a gdkconfig.h file, in similar way as GLib's

Configure now generates a gdkconfig.h file, in similar way as GLib's
configure generates glibconfig.h. Currently it only defines what
windowing API GDK uses, and on Unix (where configure scripts are
used), it's always X11...

Include gdkconfig.h in gdktypes.h.

gdkconfig.h is installed in @libdir@/gtk+/include (like glibconfig.h
is in @libdir@/glib/include). (This might be debatable?) Add that to
gtk-config --cflags.

Merge in Win32 changes to gtkfontsel.c