CMake's GNUInstallDirs knows where particular Linux architectures and
distributions want to have their libraries installed. In particular,
Debian-derived "multi-arch" distributions keep their libraries in triplet
sudirectories under /lib. Other "bi-arch" distributions keep 64-bit
libraries in /lib64.
Including GNUInstallDirs and using CMAKE_INSTALL_LIBDIR and
CMAKE_INSTALL_INCLUDEDIR means that fmt's libraries and header files are
installed in the correct locations.
Tested with OpenEmbedded and on Debian GNU/Linux 9 (the special naming only
applies when installing in /usr.)
For the consumer it should not matter if fmt has been added to the
project as subdirectory or via find_package. With the alias targets
the library can be always imported via fmt::fmt.
