Top Level vs Library Cmake

[baconpaul]

In #296 #290 and #291 among other things we see a set of issues that come from this question: Is our main CMakeLists configured for a library (so included in a higher level project) or a standalone build a wrapper (so you can build this project and get something useful).

The answer have right now is kind of not right for either.

So some staring at the CMake is order.

I would like to go to a mode where

add_subdirectory(libs/clap-wrapper)

works and also where

cmake -Bbuild -DCLAP_WRAPPER_BUILD_WRAPPER=TRUE

works but where a top level build without BUILD_WRAPPER=TRUE doesn't try to add an endpoint targets.

But what I really want to do is stare at it for a bit, come up with a plan, propose it cleanly in this issue, then get @defiantnerd and @mthierman and others to look at that plan, and if we all agree on it, then code it up for 0.11 or 0.12