clang/wrapper cannot build GHC head with integer-gmp on Mavericks

[Kazu Yamamoto (=?iso-2022-jp?B?GyRCOzNLXE9CSScbKEI=?=)]

> are you sure gmp/ghc.mk is removed? After all, it's a file checked into Git:

Oh. Probably I was confused.

So, should we introduce a hack for Mavericks to gmp/ghc.mk?

--Kazu