Andrew,
Sorry, for not testing all those combination, I tend to just add/update new software
as some program ask for it. And being Slackware OS can be moved both ways (up/downgrade) equally easy.
No problem -- testing of all the distros is most easily done here when we do the monthly builds.  In fact we were quite surprised at the Slackware build working so well here.

I think obvious fix for nasm problem will be testing nasm version in cinelerra-5.1/configure.ac
(like lines 464/465 currnetly do, but for something specific for 2.13 and up) and
disable libdav1d with message if nasm too old.

nasm --version
NASM version 2.13.03 compiled on Mar 11 2018

Or may be compile nasm as part of Cin-GG build system, but I'm afraid this way will lead to Cin OS :}
That is funny! a Cin OS! 

Also, FYI - we did contact dav1d software guru - JBKempf - and gave him our compliments and stated our concerns which he considered.  Just wanted to let him know that one of our biggest concerns was the impediment other software packages may have incorporating libdav1d into their packages for the same reasons.  But in the end, like most developers, he prefers to move forward with the latest. 

We still think it is worth putting into Cinelerra and hope that ffmpeg eventually adds an encode speed up too.  Phyllis/GG