Draft: tango.pc: fix libdir if CMAKE_INSTALL_LIBDIR is absolute

The current tango.pc.cmake file incorrectly assumes that CMAKE_INSTALL_LIBDIR is a relative path to CMAKE_INSTALL_PREFIX. If this is not the case, a path incorrectly interpolated onto CMAKE_INSTALL_PREFIX will end up in the pkg-config file.

Please read https://github.com/jtojnar/cmake-snips#assuming-cmake_install_dir-is-relative-path for more information

Closes #1578 (closed).