OpenCL support is compiled in by default, but must be enabled at runtime by an
--opencl command line flag. Compiling OpenCL support requires perl. To avoid
the perl requirement use: configure --disable-opencl.

When enabled, the lookahead thread is mostly off-loaded to an OpenCL capable GPU
device.  Lowres intra cost prediction, lowres motion search (including subpel)
and bidir cost predictions are all done on the GPU.  MB-tree and final slice
decisions are still done by the CPU.  Presets which do not use a threaded
lookahead will not use OpenCL at all (superfast, ultrafast).

Because of data dependencies, the GPU must use an iterative motion search which
performs more total work than the CPU would do, so this is not work efficient
or power efficient. But if there are spare GPU cycles to spare, it can often
speed up the encode. Output quality when OpenCL lookahead is enabled is often
very slightly worse in quality than the CPU quality (because of the same data
dependencies).

x264 must co...