[MLton] Warnings in MLton build

Just a heads up that porting MLton to other architectures turned up a
few warnings. I don't intend to fix them for debian/stretch, because
any change I make now will prevent us from making it in for the freeze
deadline.

Here are the warnings:
https://qa.debian.org/bls/packages/m/mlton.html

The array-bounds on freebsd and pointer-cast-size-mismatch are the
ones that are relevant here.

You can also see the regression results for every architecture here
(click on all and then Maybe-Successful):
https://buildd.debian.org/status/package.php?p=mlton&suite=unstable

All of the world tests failed, as expected. The real and sizes tests
need overrides for a few of the architectures.

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