Drop {INCLUDE,LIBRARY,RUNTIME}_INSTALL_DIR variables in CMakeLists.txt (#4979)

Drop {INCLUDE,LIBRARY,RUNTIME}_INSTALL_DIR variables in CMakeLists.txt (#4979)

On Debian (for instance), libraries aren't installed into `/usr/lib/`, but into something like `/usr/lib/x86_64-linux-gnu/`. In particular, this means that setting the `LIBRARY_INSTALL_DIR` to `lib` in the top-level `CMakeLists.txt` file is wrong.

Luckily, there is a simple solution: CMake provides [`CMAKE_INSTALL_LIBDIR`](https://cmake.org/cmake/help/v3.0/module/GNUInstallDirs.html) for this very purpose, which has sensible defaults and can be set by the user. In particular, since `CMAKE_INSTALL_LIBDIR` is a standardized variable, tools like the ones used for building Debian packages can set it to what they want it to be, whereas using a custom variable like `LIBRARY_INSTALL_DIR` wouldn't work in this setting.

Signed-off-by: Fabian Wolff <fabi.wolff@arcor.de>