The common modules are mostly static functions which are included
directly by the main API modules (rmr.c and rmr_nng.c). The 
static approach is to isolate them from the user application. Code
is reused as though it were linked from a library, but is only
available to our code. 

Wormholes was designed to be compiled on its own and included 
in the library as it contains the wormhole API functions, however
Cmake seems unable to generate (easily) two, differently named,
object modules from the same source, so we resort to including
it directly as though it were a static module. The symtab code
is a standalone compile, and is also available to the user 
application.