release build fails due to mesa-libs python version mismatch

[David Boyd]

When attempting to build releng/11.2 (11.2-RELEASE-p6) via the
release.sh process, the build fails with the following message:

"mesa-libs-18.1.9 needs Python 2.7 at most, but 3.6 was specified"

I was attempting to build using PORTBRANCH="ports/branches/2018Q4".

Building with PORTBRANCH="ports/branches/2018Q3" works normally.

The release process also fails if PORTBRANCH="ports/head at rHEAD" is
used.

Specifying NOPORTS=yes also allows the build to complete normally, but
forces NODOC.

Thanks, in advance, for any help that you may be able to provide.