Make ffmpeg toggleable.

--- configure.orig	2017-05-20 16:53:55 UTC
+++ configure
@@ -314,6 +314,9 @@ for arg; do
 	sun=*)
 		get_option_value enable_sun "$arg"
 		;;
+	ffmpeg=*)
+		get_option_value enable_ffmpeg "$arg"
+		;;
 	help|-h|--help)
 		error "see the INSTALL file for more information"
 		;;