When compiling on linux, use GNUInstallDirs to automatically determine
the appropriate libdir path for multi-arch builds instead of hardcoding
lib/ or lib64/. GNUInstallDirs should follow the FHS appropriately for
non-debian platforms that place libs in lib64/. This also removes the
need to specify `lib64` on the command line when building for a
non-debian system that follows the linux FHS.

When compiling with cmake directly on debian and specifying
`-DCMAKE_INSTALL_PREFIX=/usr -DCMAKE_LIBRARY_ARCHITECTURE=<arch>`, the
libdir should be set to /usr/lib/<arch>/.

This is required for when I get around to writing some Haskell bindings :-)