[MLton] linking with GnuMP

[Henry Cejtin]

I suppose that I must agree that the current cruft is just too ugly as well
as not actually working, so it makes sense to just use -lgmp even if bignums
aren't used.  I don't really agree with the compile of MLton itself being
static though.  After all, if all executables need the gmp shared library
(except for static ones) then the fact that the same is true for MLton itself
seems fine.
In addition to making the MLton executable larger (not a big deal since it is
already very large) making it static would also mean that bugs in glibc would
not be fixed until a new version came out.  More of a problem.