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.
Merge pull request #1400 from jszakmeister/install-helpers-destdir

build: fix the usage of DESTDIR in InstallHelpers.cmake