Error out if vaapi is not found

Make the behaviour consistent with the other external deps.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

configure

@@ -2840,7 +2840,7 @@ for thread in $THREADS_LIST; do
 done
 
 check_lib math.h sin -lm
-disabled vaapi || check_lib va/va.h vaInitialize -lva
+enabled vaapi && require vaapi va/va.h vaInitialize -lva
 
 check_mathfunc exp2
 check_mathfunc exp2f