CMake: Fixes for yaml-cpp and generated config

[FtZPetruska]

When packaging yaml-cpp 0.8.0 on vcpkg, we met build issues with OpenColorIO due to how the package is linked.

With the release of yaml-cpp 0.8.0, the exported target changed from yaml-cpp to yaml-cpp::yaml-cpp. This only breaks when telling CMake to use the upstream package config over the Findyaml-cpp module.

On the other hand, the upstream CMake configs have always provided a YAML_CPP_LIBRARIES variable that stores the target name.

To fix this issue, I updated the Findyaml-cpp module to be compatible with the upstream config:

• If it does not already exist, add a yaml-cpp::yaml-cpp target aliasing yaml-cpp
• Set YAML_CPP_LIBRARIES to yaml-cpp::yaml-cpp

When yaml-cpp is linked, expand the YAML_CPP_LIBRARIES variable instead of using a target name, ensuring we always get a valid target.

Lastly, in the generated config, check that neither yaml-cpp and yaml-cpp::yaml-cpp are defined before calling find_dependency.

On that note, there were a few other issues with the CMake package config:

• Using if(NOT <target name>) always evaluates to true, if(NOT TARGET <target name>) should be used to check for the existence of a target.
• The loop to find the package prefix can simply be replaced by PACKAGE_PREFIX_DIR (provided by @PACKAGE_INIT@). The assumption that the config would always be in <prefix>/lib/cmake/OpenColorIO is pretty fragile as a user could overwrite CMAKE_INSTALL_LIBDIR. Similarly, vcpkg relocates CMake packages after installation to share/<package name> (while updating PACKAGE_PREFIX_DIR), also breaking that assumption.

---

An alternative I considered was to leave the Find module intact and to handle it internally as such:

ocio_handle_dependency(yaml-cpp ...)
if(NOT TARGET yaml-cpp::yaml-cpp)
  add_library(yaml-cpp::yaml-cpp ALIAS yaml-cpp)
endif()
target_link_libraries(... yaml-cpp::yaml-cpp)

And in the exported CMake config:

if(NOT TARGET yaml-cpp)
  find_dependency(yaml-cpp ...)
endif()

if(NOT TARGET yaml-cpp::yaml-cpp)
  add_library(yaml-cpp::yaml-cpp ALIAS yaml-cpp)
endif()

Let me know if you prefer this solution instead!

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: FtZPetruska / name: Pierre Wendling (d522939, 6373161, fc6b4ab)
• ✅ login: remia / name: Rémi Achard (4123d74)
• ✅ login: doug-walker / name: Doug Walker (df0b8ab)

[michdolan]

Thanks for the fix @FtZPetruska . Looks good to me.

[listout]

This somehow broke things, now I cannot compile opencolorio with yaml-cpp 0.8.0 #1858