BUG: Use ZLIB::ZLIB imported target for system ZLIB

[blowekamp]

Replace hardcoded ZLIB paths in ITKTargets and module config files with the ZLIB::ZLIB imported target. This prevents build-host paths from being baked into installed CMake config files.

Supersedes #6489 (incorrect fix — that PR relied on hardcoded paths rather than relying on the imported interface).

[hjmjohnson]

Ran the synthetic downstream-consumer matrix (6 CMake-consumption idioms × build-tree/install-tree) against this PR, main, and #6489. This PR is identical to main in all 12 cells — it modernizes to the ZLIB::ZLIB imported target without reproducing the install-tree regression that #6489 introduced (a consumer linking ZLIB::ZLIB with no own find_package(ZLIB) failed to configure against the install tree). The difference is that this PR keeps find_package(ZLIB) in both export blocks, where #6489 emptied ITKZLIB_EXPORT_CODE_INSTALL.

Minor, non-blocking nit: the PR drops the set(ZLIB_ROOT …) hint from ITKZLIB_EXPORT_CODE_BUILD but leaves the identical hint in ITKZLIB_EXPORT_CODE_INSTALL, so the two generated export blocks are now slightly asymmetric. Harmless (the install-export hint just pins the consumer to the same zlib ITK used), just flagging for consistency.

Differential matrix — this PR (D) vs main (C) vs #6489 (A)

System ZLIB ON, artifact-scored (a consumer PASSES iff its executable exists on disk):

consumer	tree	C: main	A: #6489	D: this PR
modern_libraries	build / install	PASS	PASS	PASS
legacy_usefile	build / install	PASS	PASS	PASS
config_nomodule	build / install	PASS	PASS	PASS
direct_target	build / install	PASS	PASS	PASS
components	install	PASS	PASS	PASS
components	build	FAIL-build¹	FAIL-build¹	FAIL-build¹
naive_thirdparty	build	PASS	PASS	PASS
naive_thirdparty	install	PASS	FAIL-configure ⬅ #6489's regression	PASS ✅

¹ Documented harness constant — a COMPONENTS-restricted consumer doesn't get the deep-transitive itk_zlib.h on its include path in the build tree; identical across all three variants, so it cancels in the differential. The install tree exports it fine.

naive_thirdparty is the canary: it links ZLIB::ZLIB with no own find_package(ZLIB), relying on ITK to inject the imported target. It is the only cell that moved between variants — and this PR restores it to the main baseline on both trees.

Method & reproduction

Minimal ITK built twice (-DITK_USE_SYSTEM_ZLIB=ON/OFF, -DITK_BUILD_DEFAULT_MODULES=OFF -DModule_ITKIOPNG=ON), installed, then a family of 6 fake consumers swept against both the build tree and the install tree. Export code is generated at configure time, so each variant only needs a reconfigure + reinstall (no recompile): drop the variant Modules/ThirdParty/ZLIB/CMakeLists.txt in, cmake <build-dir>, cmake --install, sweep.

• C = git show origin/main:Modules/ThirdParty/ZLIB/CMakeLists.txt
• A = COMP: Do not require applications to rediscover zlib. #6489 (ITKZLIB_EXPORT_CODE_INSTALL emptied)
• D = this PR (ITKZLIB_LIBRARIES "ZLIB::ZLIB", find_package(ZLIB) retained in both export blocks)

The self-contained harness used to produce this (driver + 6 consumer projects + the C/A/D orchestrator) is attached to this comment as a tarball for reproduction.

Caveat: this minimal sweep exercises the ImageIO/PNG path, not DCMTK (EXCLUDE_FROM_DEFAULT), which forwards ITKZLIB_LIBRARIES into an external-project -DZLIB_LIBRARIES= string. That path is unchanged in spirit here, but a full forest build would be the belt-and-suspenders confirmation for the ZLIB::ZLIB-as-string-to-subproject case.

[hjmjohnson]

The skill used to run the downstream simulation tests:

itk-test-cmake-changes-downstream-skill.tar.gz

[greptile-apps]

Greptile Summary

This PR fixes a bug where build-host ZLIB paths were baked into ITK's installed CMake config files (ITKTargets and module config files) by switching from evaluated variable expansion (${ZLIB_LIBRARIES}, ${ZLIB_INCLUDE_DIRS}, ${ZLIB_ROOT}) to the ZLIB::ZLIB imported target.

• ITKZLIB_LIBRARIES is now set to the string \"ZLIB::ZLIB\" instead of \"${ZLIB_LIBRARIES}\", so downstream consumers resolve ZLIB via the imported target rather than a build-machine path.
• ITKZLIB_SYSTEM_INCLUDE_DIRS is removed; include directories are now carried automatically by the ZLIB::ZLIB target's INTERFACE_INCLUDE_DIRECTORIES, which is the correct modern CMake idiom.
• ZLIB_ROOT is removed from both ITKZLIB_EXPORT_CODE_INSTALL and ITKZLIB_EXPORT_CODE_BUILD, letting downstream find_package(ZLIB REQUIRED) locate ZLIB on the consuming machine rather than relying on the build host's path.

Confidence Score: 5/5

Safe to merge; the change is a minimal, targeted fix that removes build-host path contamination from ITK's installed CMake config files.

The one changed file replaces evaluated variable expansion with the ZLIB::ZLIB imported target. ITKModuleMacros.cmake explicitly handles namespaced targets (line 190 regex check), so the new string value is routed correctly at module configuration time. Removing ITKZLIB_SYSTEM_INCLUDE_DIRS is correct because ZLIB::ZLIB carries INTERFACE_INCLUDE_DIRECTORIES automatically. The ZLIB_ROOT removal from export code is the actual bug fix — it stops a build-machine path from being written into installed config files. No untested edge cases were introduced.

No files require special attention.

Important Files Changed

Filename	Overview
Modules/ThirdParty/ZLIB/CMakeLists.txt	Switches system ZLIB integration from hardcoded variable expansion to the ZLIB::ZLIB imported target, removing build-host path contamination in installed CMake config files. ITKModuleMacros.cmake already handles namespaced targets (matching ^(.*)::(.*)$) at line 190, so the new value is used correctly.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant BuildHost as Build Host (ITK configure)
    participant ExportCode as Installed ITKConfig.cmake
    participant Consumer as Downstream Consumer

    Note over BuildHost: ITK_USE_SYSTEM_ZLIB=ON

    rect rgb(255, 200, 200)
        Note over BuildHost,Consumer: Before (broken): build-host paths baked in
        BuildHost->>ExportCode: set(ZLIB_ROOT "/build/host/path") + find_package(ZLIB REQUIRED)
        ExportCode-->>Consumer: Hardcoded /build/host/path/libz.so — breaks on different machine
    end

    rect rgb(200, 255, 200)
        Note over BuildHost,Consumer: After (this PR): portable imported target
        BuildHost->>ExportCode: "find_package(ZLIB REQUIRED), ITKZLIB_LIBRARIES = ZLIB::ZLIB"
        ExportCode->>Consumer: find_package(ZLIB REQUIRED)
        Consumer->>Consumer: Resolves ZLIB::ZLIB on own system — works portably
    end

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sequenceDiagram
    participant BuildHost as Build Host (ITK configure)
    participant ExportCode as Installed ITKConfig.cmake
    participant Consumer as Downstream Consumer

    Note over BuildHost: ITK_USE_SYSTEM_ZLIB=ON

    rect rgb(255, 200, 200)
        Note over BuildHost,Consumer: Before (broken): build-host paths baked in
        BuildHost->>ExportCode: set(ZLIB_ROOT "/build/host/path") + find_package(ZLIB REQUIRED)
        ExportCode-->>Consumer: Hardcoded /build/host/path/libz.so — breaks on different machine
    end

    rect rgb(200, 255, 200)
        Note over BuildHost,Consumer: After (this PR): portable imported target
        BuildHost->>ExportCode: "find_package(ZLIB REQUIRED), ITKZLIB_LIBRARIES = ZLIB::ZLIB"
        ExportCode->>Consumer: find_package(ZLIB REQUIRED)
        Consumer->>Consumer: Resolves ZLIB::ZLIB on own system — works portably
    end

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