[CMake] Recursively include target include directories only

[Dan Liew]

> The reason I'm asking this question is because of how I handle unit
> tests in CMake right now. Instead of just defining an executable
> target for the test and then adding a link dependency on the library
> containing the class or set of classes to be tested, I am manually
> pulling in the CPP and H file into the test target and building them
> inline with the test code. This is to support mocking (I can't mock
> objects already compiled into a static library). As such, I need the
> transitive includes and defines, but I do not want the transitive link
> libraries.

Okay with that context what I suggested isn't going to completely
solve the problem.

It is actually possible to link with your static library and a mock
implementation by making the symbols that you want mock in the library
weak. Then when you link the linker will choose the strong (the mock)
implementation in preference to the weak implementation. It probably
isn't desirable to have weak symbols in a shipping library just to
make unit testing easier so you could build two versions of the
library (one with weak symbols and one without).
This would allow to use target_link_libraries() but not get link
conflicts between your mock implementation and the real
implementation.

However Petr's suggestion also sounds good (although I haven't tested it).