ENH: GDCM skip find_package when *_LIBRARIES pre-set

[blowekamp]

Use ITK module interface targets for GDCM system libraries.

Replace the EXPAT_INCLUDE_DIR/EXPAT_LIBRARY (singular) hint variables
with EXPAT_LIBRARIES/ZLIB_LIBRARIES pre-set to ITK::ITKExpatModule
and ITK::ITKZLIBModule. Guard each find_package call in the GDCM
source with if(NOT <pkg>_LIBRARIES) so a parent project that pre-sets
the plural _LIBRARIES variable skips the find entirely — the same
pattern used in the ITK DCMTK third-party module.

AI assistance

• Tool: GitHub Copilot (Claude Sonnet 4.6)
• Role: identified the skip-find pattern from InsightSoftwareConsortium/DCMTK@7c7dc11, applied it to GDCM ZLIB/Expat/OpenJPEG blocks, and drafted the commit

[blowekamp]

Updating GDCM before rebasing these changes: #6520

[greptile-apps]

Greptile Summary

This PR updates the GDCM third-party CMake setup to rely on ITK module targets.

• Expat and ZLIB library variables now point at ITK interface targets.
• OpenJPEG discovery is skipped when libraries are already supplied.
• JPEG Turbo linking now uses the ITK JPEG module target.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• The changed dependency paths still link the relevant ITK interface targets.
• The reviewed include paths are provided through those targets for the ITK build path.

Important Files Changed

Filename	Overview
Modules/ThirdParty/GDCM/src/CMakeLists.txt	Switches GDCM dependency hints from explicit include/library variables to ITK module interface targets.
Modules/ThirdParty/GDCM/src/gdcm/CMakeLists.txt	Adds guarded OpenJPEG package discovery when no OpenJPEG libraries are pre-set.

_{Reviews (1): Last reviewed commit: "ENH: GDCM skip find_package when *_LIBRA..." | Re-trigger Greptile}