backport fix for mxcl/homebrew#21259 to gmp4

gmp 4 fails to build on 10.9/Xcode 5.0.1, much like gmp 5 did. This PR backports mxcl/homebrew@8c1f197651810c50222677b611b5672388b4674e. See mxcl/homebrew#21259 for the original issue. Upstream patch is here: http://gmplib.org:8000/gmp/raw-rev/6cd3658f5621