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

[Magnus Holmgren, Millnet/Lysator/Debian/Mensa @ Pike developers forum]

The build of 7.8 that worked was with gcc-6 6.2.0-4, which according
to the changelog is actually "SVN 20160914 (r240133, 6.2.1) from the
gcc-6-branch." The build of 7.8 that failed was with 6.2.0-11, which
is "SVN 20161103 (r241817, 6.2.1) from the gcc-6-branch".