use CMAKE_INSTALL_FULL_<dir>

[dotlambda]

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

[slayoo]

Thank you!
Is there a way to test it on our CI builds? (so that we can show how it fails now, and passes after introducing this change?)

[dotlambda]

I think it might be sufficient to set -DCMAKE_INSTALL_LIBDIR to an absolute path and check the resulting pkg-config file.