Add a arg to configure to switch from GTK2 (default) to
GTK3 (optional) build for QEMU.

  ./configure --with-gtkabi=3.0

will choose GTK3, while

  ./configure --with-gtkabi=2.0

will choose GTK2 (and remains the current default)
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>
Message-id: 1361805646-6425-12-git-send-email-berrange@redhat.com
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>