Update the Mesa ports to 13.0.6

[rezny]

Summary

Small update before the end of the quatrter, wait until after the branch for the jump to 17.x. The version of LLVM used can now be overridden from make.conf (PR 217016) and use of LLVM is excluded on platforms where it is known not to be available (PR 216944). The regex used to patch configure has ben adjusted after Jan advised that [:graph:] was incorrect with his upstream submission to use [:space:] in the future.

Test Plan

Poudriere 10.3/11.0 amd64/i386 OK, builds on 11-stable ppc64, run verified on 11 amd64

[rezny]

Refresh to reset portrevision on update after jrm bumped the revisions for libpthread-stubs

[swills]

Approved