Fix invalid CMAKE_INSTALL_INCLUDEDIR concatenation.

[chpatrick]

See: https://github.com/jtojnar/cmake-snips#assuming-cmake_install_dir-is-relative-path

This fixes importing the CMake config on NixOS.

[CLAassistant]

All committers have signed the CLA.

[imciner2]

Thank you for spotting this. We don't merge directly into the master branch though, and we are currently working on the develop-1.0 branch for the next release. Can you retarget this PR to be against the develop-1.0 branch and its CMake system (we have made some changes to how the CMake is organized).?

[chpatrick]

Done, please take a look.

[imciner2]

Thanks for rebasing this. I have one question now about how this fits into the new CMake flow we have in the develop-1.0 branch. In that branch we now use configure_package_config_file to configure a CMake config file that we install on the system, so then any projects using CMake can get the libraries through this. According to the CMake documentation (https://cmake.org/cmake/help/latest/module/CMakePackageConfigHelpers.html), this should remove the absolute paths and make a relocatable package. Does this work on the Nix system? Do we need to have this change in addition to the osqp-config.cmake file?

[chpatrick]

I just checked, the develop-1.0 without this change still produces:

  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}//nix/store/z58s4i1nbyy9cmnh6bj40icdkpfnvkkl-osqp-0.6.2/include/osqp"

in osqp-targets.cmake, which isn't right.

If I use "$<INSTALL_INTERFACE:${CMAKE_INSTALL_FULL_INCLUDEDIR}/osqp>", I get:

  INTERFACE_INCLUDE_DIRECTORIES "/nix/store/glr55pijnwjkz8bxpnby21807gjw94q1-osqp-0.6.2/include/osqp

Which works on Nix (and I guess on Linux in general) but probably isn't relocatable.

If I set "$<INSTALL_INTERFACE:$<INSTALL_PREFIX>/include/osqp>", I get:

  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include/osqp"

Which I suppose is relocatable. Would that work for you?

[imciner2]

Thanks for the experiments, that is definitely useful. What is concerning to me though is the fact we then have to hardcode the include in the path in the third example, which goes against the idea of using the GNUInstallDir package and the install command into CMAKE_INSTALL_INCLUDEDIR later in the file. I did a bit of digging in the docs/issue tracker and found this comment: https://gitlab.kitware.com/cmake/cmake/-/issues/20250#note_685387. It does agree with your statement that we shouldn't use the prefix in that interface statement, but it suggests to instead just leave it out. Can you try instead just removing the $<INSTALL_PREFIX>/ from the interfaces and seeing if that makes it relocatable and work on Nix?

[notalltim]

I would love to get this fixed to clarify @imciner2 you would like "$<INSTALL_INTERFACE:osqp>"?

[imciner2]

I would love to get this fixed to clarify @imciner2 you would like "$<INSTALL_INTERFACE:osqp>"?

My concern is still the relocatability of the pathing. Can you run the experiments on Nix to see if that does fix the issue?

[notalltim]

created a new pr #567 because I added another change that was required for nix to work