PyImath CMake targets are not relocatable

[Simon Boorer]

The PyImath CMake targets are not relocatable because they add Boost_INCLUDE_DIR to the INTERFACE_INCLUDE_DIRECTORIES:
https://github.com/AcademySoftwareFoundation/Imath/blob/main/src/python/config/ModuleDefine.cmake#L39-L41

From https://cmake.org/cmake/help/latest/prop_tgt/INTERFACE_INCLUDE_DIRECTORIES.html#creating-relocatable-packages:

Note that it is not advisable to populate the INSTALL_INTERFACE of the INTERFACE_INCLUDE_DIRECTORIES of a target with absolute paths to the include directories of dependencies. That would hard-code into installed packages the include directory paths for dependencies as found on the machine the package was made on.

The INSTALL_INTERFACE of the INTERFACE_INCLUDE_DIRECTORIES is only suitable for specifying the required include directories for headers provided with the target itself, not those provided by the transitive dependencies listed in its INTERFACE_LINK_LIBRARIES target property. Those dependencies should themselves be targets that specify their own header locations in INTERFACE_INCLUDE_DIRECTORIES.

The resulting ImathTargets.cmake contain hard-coded paths to the Boost include directories:

set_target_properties(Imath::PyImath_Python3_10 PROPERTIES
  INTERFACE_COMPILE_DEFINITIONS "IMATH_DLL"
  INTERFACE_COMPILE_FEATURES "cxx_std_14"
  INTERFACE_INCLUDE_DIRECTORIES "C:/src/boost-py310-win-x64-d_e562/include"
  INTERFACE_LINK_LIBRARIES "Imath::Imath"
)

It also prevents building when Boost is contained in the source directory:

CMake Error in src/python/PyImath/CMakeLists.txt:
  Target "PyImath_Python3_10" INTERFACE_INCLUDE_DIRECTORIES property contains
  path:

    "C:/src/Imath/artifacts/Stevedore/boost-py310-win-x64-d_e562/include"

  which is prefixed in the source directory