From Alberto Luacus, "The ffmpeg plugin fails to compile with the upcoming libav 0.8 because

the required header mathematics.h is not being included explicitly.

I have just included it, and verified that this also works with current
version 0.7, since the header is also available there."

src/osgPlugins/ffmpeg/FFmpegHeaders.hpp

@@ -12,6 +12,7 @@ extern "C"
 #include <avcodec.h>
 #include <avformat.h>
 #include <avdevice.h>
+#include <mathematics.h>
 
 #ifdef USE_SWSCALE    
     #include <swscale.h>