libx265: Enable 12-bit encoding

The configure detection is bumped to X265_BUILD >= 68,
since API version 68 corresponds with the x265 1.8
release tarball. The warnings inside x265 about
12-bit being experimental were removed prior to API
version 72 a short time later.  At this time of
writing, X265_BUILD is at version 80.

12-bit support in the HEVC standard was approved in
October 2014 as part of HEVC Version 2 and published
in January 2015:

http://www.itu.int/ITU-T/recommendations/rec.aspx?rec=12296
http://www.itu.int/rec/T-REC-H.265-201410-S
https://hevc.hhi.fraunhofer.de/rext



Reveiwed-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>