print/tex-xetex: Avoid mixing libc++ and libstdc++

This port was not respecting the CC and CXX variables, thus it could
pick up gcc and g++ instead, and that would cause mixups between
libstdc++ (which is preferred by gcc) and libc++ (which is in base),
ultimately leading to link errors.

Appproved by: maintainer timeout
PR: 234221
MFH: 2021Q2