Use GNUInstallDirs to determine lib install path

[h3xx]

Lets the user specify a path to install the library in by using -DCMAKE_INSTALL_LIBDIR=foo or even =/full/path/foo.

The default of lib stays the same.

More information about GNUInstallDirs: https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html