How to handle a transitive dependency conflict using Git submodules and CMake?

There are several approaches for detect and discard inclusion of the project, which has already be included in some other parts of the main project.

Check project's target existence

The simplest pattern for single inclusion of subproject is checking existence of some subproject's target:

# When include 'C' subproject
if(NOT TARGET library_C)
    add_subdirectory(C)
endif()

(Here we assume that project C defines target library_C.)

After such conditional inclusion all subproject's targets and functions will be immediately available for the caller with garantee.

It is better to use this pattern in all places (in executable_A and library_B). Such a way changing order of library_B and library_C in executable_A doesn't break correctness.

This pattern can be reworked for use by subproject itself:

# At the beginning of 'C' project
cmake_minimum_required(...)
if(TARGET library_C)
    return() # The project has already been built.
endif()

project(C)
...

---

Check project existence

When a project is created, CMake defines several variables for it, and <PROJECT-NAME>_BINARY_DIR is among them. Note, that this variable is cached, so when cmake is called the second time (e.g. if some of CMakeLists.txt has been changed), the variable exists at the very beginning.

# When include 'C' subproject
if(NOT C_BINARY_DIR # Check that the subproject has never been included
    OR C_BINARY_DIR STREQUAL "${CMAKE_CURRENT_BINARY_DIR}/C" # Or has been included by us.
)
    add_subdirectory(C)
endif()

This pattern can be reworked for use by subproject itself:

# At the beginning of 'C' project
cmake_minimum_required(...)
if(NOT C_BINARY_DIR # Check that the project has never been created
    OR C_BINARY_DIR STREQUAL "${CMAKE_CURRENT_BINARY_DIR}" # Or has been created by us.
    project(C)
else()
    return() # The project has already been built
endif()

CMake 3.10 and later now support:

include_guard(GLOBAL)

Analogous to #pragma once in c++.

A great book on cmake that explained so much to me is "Professional CMAKE, A Practical Guide" by Craig Scott. Available online only at: [https://crascit.com/professional-cmake/][1]

I don't Craig, but his book is a great service to cmake newcomers like myself.