Log Message:
Revert previous and replace with a Darwin-specific workaround.

As joerg@ pointed out, this is not a cwrappers problem. Indeed, the
package had not been broken on NetBSD 7.x. Not sure how I jumped to my
wrong conclusion.

Instead, have the package instruct the cpp wrapper to run "/usr/bin/cpp"
on Darwin rather than the usual "clang -E". This is still a little odd,
but much less wrong, and fixes the Darwin build just as well.