[MLton] linking with GnuMP

[Matthew Fluet]

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.  It was pointed out to me
that (one of?) the MacOS X package management systems (Fink) provides
GnuMP as a package, but it installs it under its own directory, so the
(hard-coded) /usr/local/include/gmp.h doesn't find the file.  We point out
that the bin/mlton script might need to be modified to point to the gmp
lib, but we ought to set things up so that one can add a -I to the cc-opt
to point to gmp.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).