Use correct library path on all systems

- Fedora/RHEL uses /.../lib64/ for 64-bit and /.../lib/ for 32-bit
- Others use /.../lib/<arch>/
cmake: Added CMAKE_INSTALL_LIBDIR fallback and misc cleanup

The fallback is required for GnuInstallDirs to work on Ubuntu systems
configured for CMAKE_INSTALL_PREFIX set to /usr/include.

Cleanup:
  - Replaced LIB_INSTALL_DIR with CMAKE_INSTALL_LIBDIR
  - Removed TODO and unused items