Several points I noticed:

   1. The wrksrc-line is unneccessary, that is the default
   2. The makedepends-line is not terminated correctly and it is missing 
   the "ffmpeg-devel" package
   3. Build in the do_build() function, not in pre_build()
   4. Usually, calling make yourself like this is unneccessary. It is done 
   automatically by setting the build_style to gnu-make. However, it won't 
   compile like this without a little help:
   

do_build() {
        CFLAGS="-O2 -std=c99" make
}