[MLton] linking with GnuMP

[Stephen Weeks]

> While we're on the subject of linking with GnuMP, we ought to clean
> up the references to gmp.h in the platforem/*.h files.

I replaced them all with a single #include "gmp.h" in platform.h.

> I note that the BSD .h files also have absolute paths to gmp.h.  I
> think this is o.k. for packaged distributions of MLton, where we can
> add the dependence on the gmp package (and, hence, know where it
> gets installed), but for .tgz distributions, we ought to ensure that
> fiddling with /bin/mlton is sufficient to find gmp (in both .h and
> .so/.a forms).

Now that the #include is fixed, it looks like we're in good shape.  In
the 20041109 release, the packaged distributions (FreeBSD, Linux)
include a dependency on a gmp package, while the t[bg]z distributions
(Cygwin, Darwin, NetBSD, OpenBSD, Solaris) include their own copy of
gmp.h and libgmp.a.