x264: build with OpenCL lookahead support by default.

The necessary headers are included in the x264 source, so the only new dependency is perl.

The OpenCL-accelerated lookahead is disabled by default and can be enabled at runtime using the "opencl" advanced x264 option.



git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5628 b64f7644-9d1e-0410-96f1-a4d463321fa5