Consolidated INSTALL updates since _92

Consolidated INSTALL updates since _92

I've incorporated the various suggestions for INSTALL.  Here's
the current version.  (I took the opportunity to regularize some
of the malloc-related sections as well as add the appropriate
info.)

Note:  There's one error here:  I refer users to L<perlvar> for
the description of $^M, but I don't see it there!

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