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

[Marcus Comstedt (ACROSS) (Hail Ilpalazzo!) @ Pike (-) developers forum]

With another application of "handpåläggning", I completed a build of
the Debian-patched gcc 6.3 snapshot.  However, even with this I am not
able to reproduce the post-modules build failure...