configure: Fix non-fatal PKG_CHECK_MODULES invocations
configure: Fix non-fatal PKG_CHECK_MODULES invocations

Some invocations of PKG_CHECK_MODULES were intended to be non-fatal if
the package is missing, but action-if-not-found was given as an empty
string which still causes the default action to run, which halts
execution.

Signed-off-by: Scott D Phillips <scott.d.phillips@intel.com>

https://bugzilla.gnome.org/show_bug.cgi?id=769237

01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi