CMake: Add more/better GNUInstallDirs variables usage

1. Use more of GNUInstallDirs' installation location variables: INCLUDEDIR
2. Don't assume that prepending CMAKE_INSTALL_PREFIX to any of the location variables makes sense. There is no guarantee that install locations are relative to the prefix (see CMAKE_INSTALL_<location> documentation) - forcefully prepending the prefix to them can generate nonsense. CMAKE_INSTALL_FULL_<location> is what should be used to get a guaranteed-absolute version of the locations.