Checked into GIT the multibit patch for x265 in subdirectory cinelerra-5.1/thirdparty as x265_multibit_compile.patch (after modify 3.4 to 3.5 and quite a bit of testing too.  Because it takes so loooooooooooooooong to compile 

and the number of users wanting 10 and 12 bit, it was added as a patch to be applied as wanted by the user.  Instructions of how to do this have been added to the manual.  I plan on supplying a multibit AppImage version each time there is a new release at the end of the month.  Manual update (Andrea please review as I did check it into GIT, but it is quite terse):

1.8 Multibit build for 8/10/12-bit Handling
To build a version that can handle 8 bit, or 10 bit, or 12 bit videos, a patch is provided in the thirdparty subdirectory that needs to be applied to do so. Be aware
that the compile will take a substantial amount of extra time. To apply the required patch:

cd location of your cinelerra/cinelerra-5.1/thirdparty
patch < x265_compile_multibit.patch
mv x265_3.5.patch* src/.

Render formats h265-10bit and h265-12bit have been provided and will be operational after the applied patch is compiled in.

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 ..