[RFC] build: fix the usage of DESTDIR in InstallHelpers.cmake

It turns out that `file(INSTALL ...)` already accounts for `DESTDIR`, so
this wasn't creating the directory structure in the correct location.
Instead, we need to do our existence check with `DESTDIR`, but leave it
off when doing the install step.

While we're at it, add a check to make sure `ENV{DESTDIR}` is not being
used with a relative path, as that construct doesn't make much sense.

This fixes issue #1387 discovered while trying to make helptag
generation work correctly in #1381.
I see now. I think this comment is clear:

    # file(INSTALL ...) implicitly respects DESTDIR, but EXISTS does not.

A lot of confusion arises from not knowing what variables are defined by us, cmake, or make...