Set RPATH to $ORIGIN so shared objects can be found after installation

[roryatario]

This is a bare-bones commit that allows wheels to work on Linux. It needs some CMake fixes, e.g., gating the extra flag on CMAKE_SYSTEM_NAME being equal to Linux.

I believe there's another way to achieve the same thing, namely adding an __init__.py (or similar) and using ctypes to preload the SOs, since they will be in a fixed location relative to that Python source file.

While I think the auditwheel package might do something similar to (or better than!) this, it doesn't support ARM platforms, and we need pydnp3 on a Raspberry Pi-based system.