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
}