If we're using the system's installed GMP, we don't want to be picking
up the local gmp.h header file.

Fixes 2469(ghci) for me, because it turns out the system's GMP is more
up-to-date than GHC's version and has a fix for more recent versions
of gcc.  We also need to pull in a more recent GMP, but that's a
separte issue.