RPATH handling and installation paths fix

[eessmann]

Hi Tyson
This code adjusts the install interface paths to use CMAKE_INSTALL_INCLUDEDIR and adds RPATH handling using the helper function setup_quest_rpath. This ensures consistent RPATH behaviour across platforms.
Erich

[TysonRayJones]

Wew thanks very much for looking at this! Could you PR this instead to the devel branch? Then we can easily test it with changes made to the examples building, and bundle it with a v4.1 merge into main.

[eessmann]

Sure, no problem
The RPATH fix should clear up some issues I have been having on my quest rust bindings side project

[TysonRayJones]

Thanks very much! Btw do you think it's worth testing execution of the executables compiled during CI? The compiled examples (at least, those in examples/isolated) run in mere seconds, but could reveal a build issue like the bundling of the DLLs (as per #592).

If that's worthwhile, we could bundle the workflow change into this PR to test these build changes "for free". It should be as simple as adding code like below to the bottom of compile.yml:

      # attempt to run the compiled executable, testing e.g. DLL location
      - name: Run
        if: ${{ matrix.os == 'windows-latest' }}
        shell: bash
        run: |
          ./${{ env.build_dir }}/examples/isolated/Release/complex_arithmetic_c.exe
      - name: Run
        if: ${{ matrix.os != 'windows-latest' }}
        run: |
          ./${{ env.build_dir }}/examples/isolated/complex_arithmetic_c

If you think it's a good idea, I'll PR to your rpath-fix branch and we can squash-commit this PR together. Alternatively, you can make the above above edits yourself to compile.yml, though that might prove a headache; it will require my manually re-triggering the CI (auto-permissions disabled until the paid runners aren't on my credit card hehe), and experiencing the general horror of tinkering with Github Actions.

[eessmann]

I think it's a great idea to run an isolated exe for a few secs in CI. It should catch most link time errors