Segfault generating src/post_modules/GL/auto.c on PowerPC

Well, the build of gcc-6.2 itself failed with a segfault during stage2:

/var/tmp/portage/sys-devel/gcc-6.2.0-r1/work/gcc-6.2.0/gcc/tree-ssa-loop-niter.c: In function ¡void estimate_numbers_of_iterations()¢: /var/tmp/portage/sys-devel/gcc-6.2.0-r1/work/gcc-6.2.0/gcc/tree-ssa-loop-niter.c:3914:1: internal compiler error: Segmentation fault estimate_numbers_of_iterations (void) ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Please submit a full bug report, with preprocessed source if appropriate. See https://bugs.gentoo.org/ for instructions. Makefile:1087: recipe for target 'tree-ssa-loop-niter.o' failed

So I think we can simply declare gcc6 not ready for production use...