ENH: Use BUILD_INTERFACE generator expression for Python includes

[Copilot]

CMake configuration fails when Python include directories are within the source tree (e.g., .pixi/envs/python/include):

CMake Error: Target "ITKBridgeNumPyModule" INTERFACE_INCLUDE_DIRECTORIES 
property contains path "/path/to/ITK/.pixi/envs/python/include/python3.13" 
which is prefixed in the source directory.

CMake prohibits source-prefixed paths in INTERFACE properties as they leak build internals to consumers.

Changes

Modified Modules/Bridge/NumPy/CMakeLists.txt to add Python includes directly to the ITKBridgeNumPyModule target using $<BUILD_INTERFACE:...> generator expression:

• Python includes are added after itk_module_impl() creates the target
• Uses target_include_directories() with $<BUILD_INTERFACE:${Python3_INCLUDE_DIRS}>
• Generator expression ensures paths are only used during build, not in INSTALL_INTERFACE
• Works for Python in any location (inside or outside source tree)

This localizes the fix to the ITKBridgeNumPy module where the issue originates, without modifying the general module system in CMake/ITKModuleMacros.cmake.

Implementation:

target_include_directories(ITKBridgeNumPyModule
  SYSTEM
  INTERFACE
    $<BUILD_INTERFACE:${Python3_INCLUDE_DIRS}>
)

The $<BUILD_INTERFACE:...> generator expression is the modern CMake approach for handling build-time only include directories that should not be exported to consumers.

PR Checklist

• No API changes were made
• No major design changes were made
• Added test (or behavior not changed) - build system change, existing CMake configure tests validate
• Updated API documentation (or API not changed) - CMake comments added
• Added license to new files (if any) - no new files
• Added Python wrapping to new files (if any) - no new files
• Added ITK examples for all new major features (if any) - build system fix only

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Python pixi build fails with INTERFACE_INCLUDE_DIRECTORIES error</issue_title>
<issue_description>### Description

-- Configuring done (15.2s)
CMake Error in Modules/Bridge/NumPy/CMakeLists.txt:
  Target "ITKBridgeNumPyModule" INTERFACE_INCLUDE_DIRECTORIES property
  contains path:

    "/home/matt/src/ITK/.pixi/envs/python/include/python3.13"

  which is prefixed in the source directory.

Steps to Reproduce

On main:

pixi run test-python

Expected behavior

Should be allowed.

Versions

On main following #5721 .

Additional Information

@blowekamp how do you think this should be addressed?</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Python pixi build fails with INTERFACE_INCLUDE_DIRECTORIES error #5756

---

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[dzenanz]

This does not feel like the right solution. Leaving it up to Brad.

[blowekamp]

This is the incorrect approach.

[dzenanz]

@copilot apply suggestions from comment #5756 (comment)

[Copilot]

@copilot apply suggestions from comment #5756 (comment)

I've applied the correct fix in commit 91973dc. The previous approach of modifying ITKModuleMacros.cmake was indeed incorrect. The fix is now localized to the ITKBridgeNumPy module's CMakeLists.txt, conditionally setting SYSTEM_INCLUDE_DIRS based on whether Python is inside or outside the source directory.

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[dzenanz]

@copilot Use the $<BUILD_INTERFACE: > generator expression in the Numpy module, not checking whether pixi environment is created in the source directory.

[Copilot]

@copilot Use the $<BUILD_INTERFACE: > generator expression in the Numpy module, not checking whether pixi environment is created in the source directory.

The comment appears to be incomplete. Could you please provide the full suggestion? Are you referring to using a different CMake variable or generator expression?

[dzenanz]

@copilot Apply this comment: #5756 (comment) by using the $<BUILD_INTERFACE: > generator expression in the Numpy module, not by checking whether pixi environment is created in the source directory.

[Copilot]

@copilot Apply this comment: #5756 (comment) by using the $<BUILD_INTERFACE: > generator expression in the Numpy module, not by che...

Applied in commit daebeaa. Now using $<BUILD_INTERFACE:...> generator expression to add Python includes directly to the ITKBridgeNumPyModule target. This prevents CMake errors about source-prefixed paths while keeping includes available during build. Also reverted the incorrect changes to ITKModuleMacros.cmake.

[dzenanz]

This seems wrong too. I am giving up on trying to get copilot to fix this.

[blowekamp]

Super needed by #5765