Enable static libraries in "gmp" to allow static linking of Haskell programs.

Haskell programs depend on gmp to implement the unbounded Integer type. Without
static variants of gmp we cannot build statically linked Haskell programs, what
some users have found desirable in the past. So far, we've used an override to
derive our own copy of gmp in the Haskell ecosystem, but this patch ends the
dichotomy between Haskell and the rest of Nixpkgs by enabling static gmp
libraries in the default build default.