cpphs and C++-style comments

Recent versions of cpphs also strip // comments in --strip mode, which causes problems with preprocessing Data.Array.Base, where (//) is an operator. It's necessary to strip C comments from that file, because it includes MachDeps.h, and that includes ghcautoconf.h, which being generated by autoconf has lots of C-style comments. (Though gcc's cpp strips them too -- I'm not sure how this ever worked.)