* Use GNUInstallDirs to allow changing target directories

* Multi-lib distributions need to change the libdir
  which is only portably possible with `GNUInstallDirs`.
  The current `LIB_INSTALL_DIR` solution is not portable
  and not supported upstream by Kitware.
* Multi-arch distributions such as Debian and Exherbo
  need to be able to change the bindir.

* Do not install LICENSE.txt

* Most distributions do not install license files, as they
  have all licenses stored in a central repository, in order
  not to duplicate the same file over and over.
* License files do not need to be installed for a piece of
  software to be licensed under that license.