cpphs and C++-style comments

[Ross Paterson]

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.)