Fix the build on FreeBSD 12+ by using a current version of GCC which
is already used by some of the dependencies.

This avoids build errors of the following kind:

/lib/libgcc_s.so.1: version GCC_4.6.0 required by
/usr/local/lib/gcc6/libgfortran.so.3 not found

Submitted by: maintainer (Rainer Hurling <rhurlin@gwdg.de>)
PR: 225230