I tried to update x265 lib due to possible speedup reported at

https://avidemux.org/smif/index.php?topic=19448.0 (up to 30%) 

I downloaded tar.gz from https://bitbucket.org/multicoreware/x265_git/downloads/x265_3.5.tar.gz and put it in thirdparty/src, renamed x265_3.4.patch* to x265_3.5.patch* and it worked for simple 420 8bit encode .. 

not tested on older software distribitions, but I see no cmake/nasm version bumps at least in release notes...