Use relative paths for pkg-config install directory

[azeey]

🦟 Bug fix

Summary

This makes it so that all of our libraries use relative install paths and work well in build systems that don't set CMAKE_INSTALL_PREFIX.

gz_cmake_vendor is patched with this so it can build on the ROS buildfarm (see gazebo-release/gz_cmake_vendor#4).

With this patch, running cmake -DCMAKE_ERROR_ON_ABSOLUTE_INSTALL_DESTINATION=ON -P cmake_install.cmake from the build directory of any of our libraries will succeed showing that no absolute install destination is used.

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)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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