We don't need to detect GTK ABI if GTK is disabled in general.
Otherwise we could get this warning (when host is installed with GTK ABI
version 2) even when configure with "--disable-gtk":

    WARNING: Use of GTK 2.0 is deprecated and will be removed in
    WARNING: future releases. Please switch to using GTK 3.0

CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Gerd Hoffmann <kraxel@redhat.com>
CC: Peter Maydell <peter.maydell@linaro.org>
CC: Fam Zheng <famz@redhat.com>
CC: "Philippe Mathieu-Daudé" <f4bug@amsat.org>
Signed-off-by: NPeter Xu <peterx@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Message-id: 20180409082323.29575-1-peterx@redhat.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>