ffmpegthumbnailer: fix Mountain Lion build.
ffmpegthumbnailer: fix Mountain Lion build.

This also uses the args the INSTALL file says to use, adds the head url,
and uses the release tarball instead of the source tarball.

Closes #49293.
Closes #49296.

Signed-off-by: Mike McQuaid <a17fed27eaa842282862ff7c1b9c8395a26ac320@mikemcquaid.com>

filcab/homebrew,filcab/homebrew,filcab/homebrew,filcab/homebrew