use GNUInstallDirs by default
This does not change the installation directories on most systems, and improves the libdir for Debian.
See https://cmake.org/cmake/help/v3.0/module/GNUInstallDirs.html for details.
This does not change the installation directories on most systems, and improves the libdir for Debian.
See https://cmake.org/cmake/help/v3.0/module/GNUInstallDirs.html for details.