CMake and using git-submodule for dependence projects

Question

Consider the following three projects.

ProjectA does not have any dependencies and its CMakeLists.txt at top level is like the following,

Answer 1

You can check whether the target a already exists before calling add_subdirectory:

if (NOT TARGET a)
  add_subdirectory(ProjectA)
endif ()

so that it only adds the subdirectory once for your whole CMake project.