Split the GMP build rule even further
Split the GMP build rule even further

See #289.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,bgamari/shaking-up-ghc,sdiehl/ghc,sdiehl/ghc,izgzhen/hadrian,sdiehl/ghc,sdiehl/ghc,snowleopard/shaking-up-ghc,snowleopard/hadrian