[CMake] How to get a variable that is defined in another CMakeLists.txt file

[Sylvain Benner]

You can inherit from the top level CMakelists.txt variables.
So if the GET_DIRECTORY_PROPERTY command does not work for your case, 
you can use the SET command in the top level CMakeLists.txt which calls 
the command ADD_SUBDIRECTORY of your libraries A and B.

Something like:

IF(condition)
    SET (PROJECT_A_LIB_NAME MyLib)
    ADD_SUBDIRECTORY(A_Directory)
ENDIF(condition)

Then you can safely try to read the PROJECT_A_LIB_NAME variable in the 
CMakeLists.txt of lib B.

-Sylvain