configure: Don't permit SDL or GTK on OSX

configure: Don't permit SDL or GTK on OSX

The cocoa GUI frontend assumes it is the only GUI (it redefines
main() so it always gets control before the rest of QEMU), so
it does not play well with other UIs like SDL or GTK. (Mostly
people building QEMU on OSX don't have the necessary dependencies
available for configure to build those other front ends, so
mostly this problem goes unnoticed.)

Make configure automatically disable the SDL and GTK front ends
if the cocoa front end is enabled. (We were sort of attempting
to do this for SDL before, but not in a way that worked very well.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: John Arbuckle <programmingkidx@gmail.com>
Message-id: 1439565052-3457-1-git-send-email-peter.maydell@linaro.org