Replace hardcoded installation directories with GNUInstallDirs.

[tastytea]

GNUInstallDirs are portable, many users are familiar with them and they make the
life of package maintainers easier.

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Checklist

Not all of these might apply to your change but the more you are able to check
the easier it will be to get your contribution merged.

• CI passes
• Description of proposed change
• Documentation (README, code doc blocks, etc) is updated
• Existing issue is referenced if there is one
• Unit tests for the proposed change

CMake compatibilty

GNUInstallDirs is available in 3.10:
https://cmake.org/cmake/help/v3.10/module/GNUInstallDirs.html.

Reasoning

Linux distributions usually have profiles for CMake that set these directories
to the right locations. This change will make packaging easier.

Other

I have left the old variables intact for backwards compatibility.

[mrtazz]

thank you!