Add some fixes for building on macOS Homebrew.
- Set ZLIB_ROOT because zlib is now keg-only. Unfortunately the cmake
warning about policy CMP0074 cannot be suppressed because of some kind
of cmake bug.
- Update FindFFmpeg.cmake from:
https://github.com/snikulov/cmake-modules/blob/master/FindFFmpeg.cmake
, this fixes a problem with FFmpeg not being found.
Signed-off-by: Rafael Kitover <rkitover@gmail.com>