[MLton] linking with GnuMP

[Stephen Weeks]

> The
> point is that if you have an SML executable which doesn't use IntInf  in  any
> way, say
>     val _ = print "Hello world0
> then  it  is  bad  (not  horrible,  but not good) if the resulting executable
> requires libgmp.so to be installed.

This reminds of another new factor pointing towards using libgmp.so:
even hello-world brings in IntInf.  If you compile hello-world -keep
ssa, you will see a call to IntInf_smallMul in the SSA.  I think this
comes from the fact that Time is now implemented using IntInf.  We
could probably find a way to work around this, but it's yet another
way in which it is harder to avoid pulling in libgmp.