Target settings do not automatically populate when using CMake FetchContent

[Andrew-Au]

MacOS using XCode

Create FetchContent import as follows:

include( FetchContent )
FetchContent_Declare(
	jansson
	GIT_REPOSITORY https://github.com/akheron/jansson.git
	GIT_TAG 684e18c927e89615c2d501737e90018f4930d6c5 # v2.14
)

FetchContent_MakeAvailable(
	jansson
)

What works:

• automatic download of Jansson repository
• automatic build of Jansson repository
• inclusion of test targets into generated XCode project

What fails:

• search path to "jansson.h" not included in build (<jansson.h> also fails)
• Jansson compile options not added to build

---

I enquired on the CMake forums and the FetchContent experts explained that the problem was the Jansson project not using per-target options. Because these are missing, the options are not exported along with the target and the build fails.

Proposed fixes:

• add_definitions(-DJANSSON_USING_CMAKE) and add_definitions(-DHAVE_CONFIG_H) become target_compile_definitions( jansson PUBLIC JANSSON_USING_CMAKE) and target_compile_definitions( jansson PRIVATE HAVE_CONFIG_H) respectively.
• include_directories( ${CMAKE_CURRENT_BINARY_DIR}/include ) becomes target_include_directories(jansson PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}/include> INTERFACE $<INSTALL_INTERFACE:include> )
• Add library alias Jansson::jansson
• Update minimum CMake to 3.5. As of 3.27 minimums below 3.5 are deprecated.

Testing on OS X with CMake 3.26

• Jansson download and build works
• App build using FetchContent now works correctly
  • Can define Jansson::jansson as dependency
  • "jansson.h" header correctly found
  • JANSSON_USING_CMAKE correctly added to compile options