The basic idea is set a minimum required gcc, g++, and gfortran. If they are
not met by the "base" compiler, then build an appropriate pkgsrc gcc package
and use that for everything that follows.
It would be nice if you were allowed to override this and use the base
compiler, at your own risk, even though it is supposedly too old. On a
legacy platform such as MirBSD, this is your best shot at getting at
least plain C packages compiled -- even though things like harfbuzz
will likely prevent you from compiling anything more fancy.