CMake - How to get include directories of external project? - cmake

Question

I tried to used https://github.com/julianxhokaxhiu/SteamworksSDKCI to use steam api on a simple SFML application (helloworld).
I wanted to use cmake to learn it, but I am struggling to understand how the provided CMakeLists and Find*.cmake file are expected to be used.
Currently, I have modified the CMakeLists to change the INSTALL_DIR
INSTALL_DIR "${CMAKE_BINARY_DIR}/../../vendor"
and my CMakeLists is :
cmake_minimum_required(VERSION 3.19)
project(SfmlWithCMake VERSION 1.0)
include(FetchContent)
set (CMAKE_MODULE_PATH "${CMAKE_MODULE_PATH};${CMAKE_CURRENT_SOURCE_DIR}/cmake_steam")
# Configure external project
file(MAKE_DIRECTORY ${CMAKE_BINARY_DIR}/cmake_steam)
execute_process(
COMMAND ${CMAKE_COMMAND} ${CMAKE_SOURCE_DIR}/cmake_steam
WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/cmake_steam
)
# Build external project
execute_process(
COMMAND ${CMAKE_COMMAND} --build ${CMAKE_BINARY_DIR}/cmake_steam
)
set(BUILD_SHARED_LIBS OFF)
FetchContent_Declare(
SFML
GIT_REPOSITORY https://github.com/SFML/SFML.git
GIT_TAG 2.5.1
)
FetchContent_MakeAvailable(SFML)
find_package(STEAMWORKSSDK REQUIRED)
set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED true)
# Generate config.h
configure_file(config.h.in config.h)
add_executable(
SfmlWithCMake
main.cpp
)
get_target_property(STEAMSDK STEAMWORKSSDK::STEAMWORKSSDK INCLUDE_DIRECTORIES)
target_include_directories(
SfmlWithCMake
PRIVATE
"${PROJECT_BINARY_DIR}"
"${STEAMSDK}"
)
target_link_libraries(
SfmlWithCMake
sfml-graphics
STEAMWORKSSDK::STEAMWORKSSDK
-static gcc stdc++ winpthread -dynamic
)
install(TARGETS SfmlWithCMake DESTINATION bin)
How to get include directories?
I do not succeed to add the steam include to the target_include_directories.
Here the ${STEAMSDK} is my last attempt to get the directory.
If I replace this by ${PROJECT_BINARY_DIR}/vendor/include, everything works.
Also, why does the SFML include are automatically added to my target include directories and not the steam one?
Am I using the Find*.cmake file the right way?
I understood that ExternalProject_Add was performed at build time and thus, as the find_package is needed at configue time, I added the two "execute_process". But, the readme on github only says to do the find package and add the target to target_link_libraries...
Thanks.