https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79771
--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> --- Do you really need even the zlib.def change? That part has been added 5 years ago, so it would surprise me if it didn't build even with that. If that works, the gzguts.h and zlib.h changes is something we can apply to gcc's copy until it is resolved upstream.
