Fix cmake abspath conversions

[rtburns-jpl]

CMAKE_INSTALL_*DIR variables are not necessarily relative
to the install prefix - they may be absolute, in which case
appending them to CMAKE_INSTALL_PREFIX is not valid.

Fixes installation paths when provided GNU_INSTALL_*DIRs are not relative to CMAKE_INSTALL_PREFIX, as used by some tooling - e.g. -DGNU_INSTALL_LIBDIR=/usr/lib/
See https://github.com/jtojnar/cmake-snips#assuming-cmake_install_dir-is-relative-path for more info.

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[seisman]

Will review this PR in the next few days.

[seisman]

It looks good, except a tiny fix.

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[seisman]

@rtburns-jpl After merging this PR, we can't build the GMT macOS bundle anymore. Please see the issue report in #4481.

It's unclear to me how CMake builds bundles, so we have to revert the changes in this PR (#4484) now.

If you know more about CMake and know how to fix it, we're glad to re-merge your changes again.