Don't assume CMAKE_INSTALL_*DIR variables are relative

[lopsided98]

🦟 Bug fix

The CMAKE_INSTALL_*DIR variables are allowed to be absolute paths, so they cannot be simply appended to CMAKE_INSTALL_PREFIX. When an absolute path is needed, CMAKE_INSTALL_FULL_*DIR must be used. See here for more information: https://github.com/jtojnar/cmake-snips#assuming-cmake_install_dir-is-relative-path

Additionally, pkgconfig files should use ${prefix} relative paths if CMAKE_INSTALL_*DIR are relative, but otherwise use absolute paths. This is handled by the custom join_paths() function.

Note that CMake 3.20 provides cmake_path(APPEND) which implements the same functionality as join_paths(), but this CMake version is not available in all supported distros yet (notably Ubuntu older than 22.04).

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
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• All tests passed (See test coverage)
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[j-rivero]

Thanks Ben for the pointers and the patch. Changes in code looks good to me, my testing a local wokrspace for packages up to gz-transport is fine and I run a testing build of the whole collection in for Windows (warnings and test are unrelated as far as I can tell).

Note that CMake 3.20 provides cmake_path(APPEND) which implements the same functionality as join_paths(), but this CMake version is not available in all supported distros yet (notably Ubuntu older than 22.04).

I'm going to add a TODO not to forget about this.