cmake: Still build with shared lib loading support, even if there's only one?

[EricCousineau-TRI]

At present, the current CMakeLists.txt seems a bit too implicit:
https://github.com/ros2/rmw_implementation/blob/c5f5324/rmw_implementation/CMakeLists.txt#L41-L44

Can this be made into an explicit CMake switch, something like -DFORWARD_RMW_IMPLEMENTATION={OFF,ON}?

Rationale:
I'm developing on a machine that only has one RMW implementation present.
In debugging stuff like ros2/rcpputils#3 and ros2/rcutils#143, it I couldn't just rebuild rmw_implementation to reproduce what I have installed as a binary.

The only way that I can reproduce the binary is either (a) modify CMakeLists.txt or (b) install an (unused) RMW implementation, which is odd... (example patch line)

I should be able reproduce this with my installed packages?

[dirk-thomas]

Can this be made into an explicit CMake switch ...?

That sounds fine. I would suggest adding a boolean CMake option like RMW_IMPLEMENTATION_FORCE_POCO. Please consider creating a pull request for this.