How does system introspection work exactly in cmake?

[automational]

I am toying with a CMake project for Apknite and in the tutorial, there is a step about system introspection.

I tried to do a very simple test which I thought would work but unfortunately, it doesn't.

I have a simple module called add which exists in its own module and defines a function called add.

directory structure

├── bin
├── cmake_bin
├── CMakeLists.txt
├── src
│   ├── add
│   │   ├── add.c
│   │   ├── add.h
│   │   └── CMakeLists.txt
│   ├── CMakeLists.txt
│   ├── main.c
│   ├── main.h
│   └── version.h.in

src/CMakeLists.txt has the following lines

add_executable(main.out main.c)
add_subdirectory(add)

include(${CMAKE_ROOT}/Modules/CheckFunctionExists.cmake)
check_function_exists (add HAVE_ADD)
configure_file(
	"${CMAKE_CURRENT_LIST_DIR}/version.h.in"
	"${EXECUTABLE_OUTPUT_PATH}/version.h"
)

where main.c includes add.h and makes use of the add function.

version.h.in contains

#cmakedefine HAVE_ADD

Can someone help me understand why when version.h is generated it reports looking for add - not found?

What I have tried so far is to move the check_function_exists in the src/add/CMakeLists.txt and to use check_symbol_exists(add add.h HAVE_ADD).

After a glance at the internetz, I saw that check_function_exists is for system header files but I don't quite get why or how it would distinguish between system files and my source files.

Any help is appreciated.