Fix typo (HAVE_LIBGMP => HAVE_LIB_GMP); omit local gmp includes if HAVE_LIB_GMP
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.
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