[PROTON-2774] Support composite CMake builds

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Incomplete
• Affects Version/s: proton-c-0.39.0
• Fix Version/s: proton-c-0.40.0
• Component/s: build, proton-c
• Assignee: Jiri Daněk
• Reporter: Jiri Daněk
• Created: 14/Oct/23 16:28
• Updated: 13/Jan/24 17:08
• Resolved: 13/Jan/24 17:08

Description

CMake recognizes the notion that project may be either top-level, or included from other projects that intend to depend on it.

https://cmake.org/cmake/help/latest/variable/PROJECT_IS_TOP_LEVEL.html

This inclusion is performed either with add_subdirectory() or by FetchContent_MakeAvailable().

There are several preconditions for this to work

#. proton must compile cleanly with whatever compile (warning) flags the top-level project sets; I intend to clean proton so that it compiles with qpid-dispatch/skupper-router flags
#. CMakeLists.txt need to be written to not assume that CMAKE_SOURCE_DIRECTORY corresponds to the currently active project. Use the PROJECT_SOURCE_DIRECTORY and similar variables instead.
#. targets in CMakeLists.txt must be written so that they can be immediately used by the including projects
#. tests and other miscellaneous parts of the project should not be enabled when proton is not the top-project

As an example, the Google Test project supports this kind of composite builds.