mediastreamer: work around a build problem

The new deprecation was most likely triggered by ffmpeg update to 2.8.*

pkgs/development/libraries/mediastreamer/default.nix

@@ -33,6 +33,8 @@ stdenv.mkDerivation rec {
     "--enable-glx"
   ];
 
+  NIX_CFLAGS_COMPILE = "-Wno-error=deprecated-declarations";
+
   meta = with stdenv.lib; {
     description = "a powerful and lightweight streaming engine specialized for voice/video telephony applications";
     homepage = http://www.linphone.org/technical-corner/mediastreamer2/overview;