To allow docs to be built for the release candidate, and in the absence of osmesa, you’ll want to apply the changesets below (already applied to default, but not to stable).
These first two fix problems with gnuplot v5
This one backs-up osmesa with gnuplot
Its not obvious that either of these qualify as regressions, so I’m not planning to merge these into stable. However, if there is sufficient testing and support for doing so, I have no objection. In the meantime, I recommend that homebrew, macports, and Fink apply each of these.
Ben