configure: do not break configure if gtk+-3.0 devel missing

configure: do not break configure if gtk+-3.0 devel missing

Fix PKG_CHECK_MODULES rule for with_gtk=check condition to
set USE_GTK=0 if gtk+-3.0 is not available.

Since commit 85856c29a70d6de4aea5b708e04e9eb418190623
Author: Hyunjun Ko <zzoon@igalia.com>
Date:   Wed Jul 5 15:59:43 2017 +0900

    tests: elements: add testsuite of vaapi context

...configure fails if gtk+-3.0 development files are missing.

The "with_gtk" option defaults to "check" in configure.ac
which implies that if it is not explicitly requested then
configure will only enable it if it's available on the system.

However, the PKG_CHECK_MODULES rule that get's activated on
"check" condition did not provide default when gtk+-3.0 devel
packages are not found on the system.  Thus, it resulted in
configure failure.

Signed-off-by: U. Artie Eoff <ullysses.a.eoff@intel.com>
https://bugzilla.gnome.org/show_bug.cgi?id=785452