[MLton] install rule

[Stephen Weeks]

> I have a question about the install rule in the main Makefile: what
> is the purpose of $(DESTDIR)?  Why is the default to install into an
> install directory?

Caveat: all that DESTDIR/PREFIX stuff in the Makefile has grown
incrementally as we've added platforms, so there may be no good reason
or answer, and there may be a better way to do things.  But, it's not
fun messing with that stuff since you then have to retest packaging on
every platform.

That having been said, DESTDIR and PREFIX are there to let the
packaging mechanism for a particular platform (debian/rules,
doc/mlton.spec, freebsd/Makefile) specify where to install the
package.  Henry may remember more about why it's set up the way it is.

Installing in a subdirectory of the source directory seems safe and
sensible, unless the user specifies to put stuff somewhere else.