Add a build mode to build the client/swig bits as sub_directories
- Optional development only build mode
- Controlled by the "COMBINED_BUILD" define on the cmake command line
- Uses add_subdirectory instead of ExternalProject
- Allows some cmake based tools to introspect the whole project better
- Can be used for faster build and test times as it allows parallel tests and builds which get blocked by the ExternalProject machinery
- Leaves the external project mechanism as the default build mode.