Add support for specifying which commit/branch of reactor-c to use

[erlingrj]

With the Cpp-target we can instruct lfc to compile the generated application with a specific commit/branch of reactor-cpp, or with reactor-cpp located at a certain location. (@cmnrd is that accurate?). Using CMake (i.e. not done from lfc), reactor-cpp is then cloned and checked out at the right commit. This would be great to support for reactor-c also. There are a few problems:

• We don't really copy reactor-c in as a submodule, rather we copy in most of the subdirectories of reactor-c to src-gen. I think this is a little odd and we should probably move to a copying the entire reactor-c into a directory in src-gen.
• This would be hard to support for Ardunio which requires modifying all include statements and removing all source files which is not to be compiled.

[cmnrd]

Yes, the runtime-version property allows setting a specific git commit ref for the reactor-cpp runtime, without modifying the submodule version. I use this a lot when developing new features in reactor-cpp. The property can both be set in an LF files target properties or using a command line argument to lfc. Leveraging the current cmake structure for the C++ target, this mechanism can even keep multiple different compiled versions and switch between them without recompiling everything.