Fix building with LLVM

Fix a plist issue [1].

Make sure the LLVM options are correctly handled. Setting the OPTIONS_DEFAULT
conditionally based on wheather the default compiler is cland doesn't work
properly, it is always set even if you deselect it. Just setting it per default
seems to work better.

Attempt a fix for building on FreeBSD-13.0 where clang is newer than the
default LLVM. First attempt is to use an lower version clang to match what LLVM
has as default. [2]

PR: 244403 [2], 249933 [2], 249875 [1]