mesa: LLVM_DEFAULT usage

[Gunther Nikl]

While testing the phabricator differential for mesa-dri/mesa-libs (D19100),
I noticed that I don't understand it's dependency on LLVM. I assumed that
the referenced LLVM version is required to build the ports, but the build
used cc from base unless I added explicitly set CC and CXX as done in the
PPC block. Only llvm-config from the referenced port is used. Is this
intentional?
I am using 11.3-RELEASE AMD64.

Thanks,
Gunther Nikl