Exit with error when running libavtest and not configured with --enable-gpl.

Originally committed as revision 9141 to svn://svn.ffmpeg.org/ffmpeg/trunk

tests/Makefile

@@ -36,6 +36,7 @@ libavtest:
 	@echo
 	@echo "This test requires FFmpeg to be compiled with --enable-gpl."
 	@echo
+	@exit 1
 endif
 
 ifeq ($(CONFIG_SWSCALER),yes)