Trying to emerge mesa fails almost immediately, on the meson configure phase with this error:

meson.build:21:0: ERROR: Value auto is not boolean (true or false).

I get this exact behavior on three machines with very different configurations, with one of the installations being tad fresh (month old)

Judging from the error, and the command line options passed to meson configure and looking at the ebuild itself, it looks like the ebuild erroneously feeds "auto" to boolean variables which can only be true or false, namely the -Dgallium-nine and -Dgallium-extra-hud options, for the d3d9 and extra-hud USE flags, respectively.

Testing with a local ebuild with those entries getting "true" instead of "auto" makes it go past this error in meson configure, but if d3d9 USE flag is still set, it then errors out with:

meson.build:647:4: ERROR: Problem encountered: The nine state tracker requires gallium softpipe/llvmpipe

Disabling the d3d9 USE flag allows it build, but from the looks of it, the ebuild needs non-trivial changes for d3d9 use flag to be usable at all.