Without this change, if you link with an 8bit libx265 and try to specify
a 10bit input color space via:

  ffmpeg -i in.mov -c:v libx265 -pix_fmt yuv420p10le out.mp4

It will error with:

  Incompatible pixel format 'yuv420p10le' for codec 'libx265',
  auto-selecting format 'yuv420p'

With this fix, it will learn if a 10bit libx265 is available at startup,
and thus allow 10bit input color spaces.
Reviewed-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>