(via Mk/bsd.default-versions.mk and lang/gcc) which has moved from
GCC 5.4 to GCC 6.4 under most circumstances.

This includes ports
 - with USE_GCC=yes or USE_GCC=any,
 - with USES=fortran,
 - using Mk/bsd.octave.mk which in turn features USES=fortran, and
 - with USES=compiler specifying openmp, nestedfct, c++11-lib, c++11-lang,
   c++14-lang, c++0x, c11, or gcc-c++11-lib.

PR:		219275