configure: don't warn GTK if disabled

configure: don't warn GTK if disabled

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: Peter Xu <peterx@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20180409082323.29575-1-peterx@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>