multimedia/x264: add OPENCL option, enabled by default

Before OpenCL can be used for lookahead install at least one
implementation: devel/freeocl, devel/pocl, lang/beignet or lang/clover.

$ x264 --fullhelp | fgrep opencl

--opencl Enable use of OpenCL
--opencl-clbin <string> Specify path of compiled OpenCL kernel cache
--opencl-device <integer> Specify OpenCL device ordinal

where the options also work in other apps e.g.,

$ ffmpeg -i <input-file> -codec:v libx264 -x264opts opencl:opencl-device=1 <output-file>
$ mpv --ovc libx264 --ovcopts-add x264opts=opencl:opencl-device=1 -o <output-file> <input-file>

Differential Revision: https://reviews.freebsd.org/D7958
Approved by: koobs (maintainer)