The error:

..
Makefile:645: recipe for target 'all-recursive' failed
make: *** [all-recursive] Error 1
..

I've read in a Gentoo forum that this is related to an older version of `llvm` or, vice versa, mesa-12.0.1 is an old version for newer versions of `llvm`.

I use Funtoo stable. As I have (had) other problems when using newer versions of various ebuilds, under stable, I am not sure how to proceed.