cpp.c: fix darwin build

Darwin doesn't support fmemopen.c, so cpp.o will not link. Googling this issue suggests that this is a common problem.

The fix included uses the fmemopen.c function from DragonflyBSD which should be compatible with the licence of 8cc.
remove duplicate import
Merge pull request #9 from davecheney/darwin-fmemopen

cpp.c: fix darwin build