Make compilation of different network channels scalable

[erlingrj]

Currently there is a single source file, network_channel.c which conditionally includes all the source files providing the different network channel implementation. This is brittle since they all go into the same compilation unit. So any global (or static) variables will be shared across the different implementations.

The scalable solution is to instead conditionally include the various source files into the build. This means moving the logic out from C code (using macros), to Makefile/CMake