configure: create .config file in configure to avoid needless reruns

Originally committed as revision 24372 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -3154,6 +3154,7 @@ print_config AV_HAVE_ $TMPH /dev/null $HAVE_LIST_PUB
 echo "#endif /* AVUTIL_AVCONFIG_H */" >> $TMPH
 
 cp_if_changed $TMPH libavutil/avconfig.h
+touch .config
 
 test -n "$WARNINGS" && printf "\n$WARNINGS"