CMake changing a library include directory based off of calling project - c++

Question

Here is my current project structure:
ProjectX
|_ projs
|_ A ( builds an external project POO version 1 )
add_executable(A ${A_HDRS} ${A_SRCS} $<TARGET_OBJECTS:libbar>)
|_ B ( builds an external project POO version 2 )
add_executable(B ${B_HDRS} ${B_SRCS} $<TARGET_OBJECTS:libbar>)
|
|_ libs
|_ libbar (objects only - no linking)
(needs POO either v1 or v2 depending on calling project)
issue lives here -->INCLUDE_DIRECTORIES(${POO_BUILD_DIR}/include)
add_library(bar OBJECT ${LIB_BAR_HDRS} ${LIB_BAR_SRCS})
GOAL:
I would like to be able to build from the top level including all projects and have libbar to be built with the correct dependencies. I would like to be able to have a continuous build process.
DETAILS:
When project A build builds, it needs to compile libbar with POO version1 and when project B builds it needs to compile libbar wtih POO version2. In either case it is still a version of POO. I would like to be able to pass the path to libbar from A or B.
ISSUE:
The POO_BUILD_DIR path is different depending on the project building libbar. When I build from the top level, make tries to build libbar first and doesn't find the correct includes.
QUESTION:
Does anyone know how to achieve this in cmake or how can I achieve finer grained control over the build order?
UPDATE and SOLUTION:
The problem that I was having could have been solved with Chris's solution below. However, my problem (that I should have realized earlier) is that even though it lives in a library directory and is called a library it's not a library. The naming and directory structure I can't change b/c I don't own the code base. However, I can change how I build it! My quote-unquote solution was to just include the files to the compilation like this: add_executable(A ... ${libbar_hdrs} ${libbar_srcs}
Moral of the Story
If someone tells you it's a library and names it a library - it still may not be. Or expressed in a colloquialism "Don't always believe what you're told".

Answer 1

Well there's lots of ways that you could do that, but one way that I have done it in a cmake C++ project is, for each library (each folder inside of /lib in my project root) I have a secondary Cmake script which builds that library, and I use add_subdirectory within my primary cmake script to invoke those secondary cmake scripts.
Here's an (open source) example from my project: https://github.com/cbeck88/cegui-emscripten
For instance, if you look here you'll see how I do "in tree" (what you are talking about) libs using add_subdirectory, vs. "out of tree" (user provided) libs which I find using find_package. https://github.com/cbeck88/cegui-emscripten/blob/17f0d097f989862035e977a6b9e0b1bbb1fcdf21/CMakeLists.txt#L58
if (FREETYPE_IN_TREE)
add_subdirectory(lib/freetype-2.5.5 freetype-2.5.5)
else()
LIST(APPEND CMAKE_PREFIX_PATH ${CMAKE_CURRENT_SOURCE_DIR}/lib/freetype-2.5.5-old )
find_package(freetype NO_CMAKE_FIND_ROOT_PATH)
MESSAGE ( STATUS "FREETYPE found = " ${FREETYPE_FOUND} )
endif()
This is what my secondary cmake script (in folder freetype-2.5.5) looks like, it will look very different in your case but at least it's an example. https://github.com/cbeck88/cegui-emscripten/blob/d24fcd6a5dc4697b8718564fadb25d76c255bce2/lib/freetype-2.5.5/CMakeLists.txt
Note especially some of these lines at the very end SET(FREETYPE_INCLUDE_DIRS ${INCL} PARENT_SCOPE) which you might need to use if you do it this way.