Library installation path ought to be modified by packagers

Currently the libraries generated by a CMake build are installed in a fixed path. A packager ought to be able to change this path in order, for example, to allow a multi-arch installation.

This is much in the same vein as with Qt/CMake's CMAKE_LIBRARY_PATH configuration

If the headers produced by the library change if built on different architectures then the include path also should be modified, like in here. But again, this is only necessary if the headers change with the architecture.