configure: don't warn SDL abi if disabled

SDL has the same problem as GTK that we might get warnings on SDL ABI
version even if SDL is disabled.  Fix that by only probing SDL if SDL is
enabled.  Also this should let configure be a little bit faster since we
don't really need to probe SDL stuff when it's off.

CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Gerd Hoffmann <kraxel@redhat.com>
CC: Peter Maydell <peter.maydell@linaro.org>
CC: Daniel P. Berrange <berrange@redhat.com>
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>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 20180410054034.20479-1-peterx@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>