CMake "Cannot specify sources for target PROJECT_NAME which is not built by this project." - cmake

Question

I am learning CMake but unfortunately most examples are either too simple, too complicated or the folder structure is different to what I'm designing.
I am getting an error but first I will explain the folder structure (please do critique):
MyProject
bin
build
src
ComponentA
ObjectA.cpp
CMakeLists.txt
ComponentB
ObjectB.cpp
CMakeLists.txt
CMakeLists.txt
main.cpp
CMakeLists.txt
I would like to be able to include some files using their absolute path, for example main.cpp might look like this:
#include <ComponentB/ObjectB.h>
int main()
{
ComponentB cb(1, 2, 3);
}
but within a source file I'd like to include it's header using the relative path:
#include <ComponentB.h>
ComponentB::ComponentB(int a, int b, int c) : _ca(a, b){}
(if this causes problems I can include using absolute paths)
My CMakeLists files look like:
MyProject/CMakeLists.txt:
cmake_minimum_required(VERSION 3.9.1)
project(MyProject)
add_subdirectory(src)
MyProject/src/CMakeLists.txt:
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_ROOT_DIR}/bin)
add_executable(MyProject main.cpp)
add_subdirectory(ComponentA)
add_subdirectory(ComponentB)
MyProject/src/ComponentA/CMakeLists.txt:
target_sources(MyProject PUBLIC ComponentA.cpp)
MyProject/src/ComponentB/CMakeLists.txt:
target_sources(MyProject PUBLIC ComponentB.cpp)
target_include_directories(MyProject PUBLIC ComponentA)
However when I do:
cd build
cmake ..
I get this this error:
CMake Error at src/ComponentA/CMakeLists.txt:2 (target_sources):
Cannot specify sources for target "MyProject" which is not built by
this project.
CMake Error at src/ComponentB/CMakeLists.txt:2 (target_sources):
Cannot specify sources for target "MyProject" which is not built by
this project.
CMake Error at src/ComponentB/CMakeLists.txt:3 (target_include_directories):
Cannot specify include directories for target "MyProject" which is not
built by this project.

Answer 1

The ability for target_sources to add source files via relative path to a target in a different directory was added in CMake 3.13. Since you have specified a minimum version of 3.9.1, this will simply not work.
The policy in question is CMP0076, which is documented as follows:
New in version 3.13.
The target_sources() command converts relative paths to absolute.
In CMake 3.13 and above, the target_sources() command now converts relative source file paths to absolute paths in the following cases:
Source files are added to the target's INTERFACE_SOURCES property.
The target's SOURCE_DIR property differs from CMAKE_CURRENT_SOURCE_DIR.
It is the second bullet point that applies here.
You really ought to use a newer version of CMake. Anything more than a few versions behind the newest (3.23) is plain masochism.