How to add external libraries - Crowdsource_pi example

[rgleason]

#420 (comment)

Crowdsource_pi Example

Yes. The solution was to not use externalproject_add, but subdirectory_add, and then modify the target properties as needed.

In general, trying to work around problems in the build system of libraries from the main cmakelists.txt is not exactly a workable solution - you kinda have to fork those projects and patch them.