x264: Check that minimum version is met
Since the addition of monochrome encoding support, we now depends on build 156. No version check was happening so far.
This is an alternative fix for !5323 (closed) I dislike infinitly growing set of ifdef as they inevitably endup unmaintainable. This is of course open to debate, if you prefer ifdef let me know and I'll merge the other version.
cc @rgonzalez