BUG: Fix itk_expat.h not found when ITK_USE_SYSTEM_EXPAT is ON

Commit bfc8e07 moved the configured itk_expat.h output from
${ITKExpat_BINARY_DIR}/src/ to ${ITKExpat_BINARY_DIR}/ and updated
the vendored (ITK_USE_SYSTEM_EXPAT=OFF) include path to match, but
the system-expat branch still pointed at the old src/ subdirectory.

This caused a fatal error in gdcm_expat.h when building with
ITK_USE_SYSTEM_EXPAT=ON (or ITK_USE_SYSTEM_LIBRARIES=ON):

  fatal error: 'itk_expat.h' file not found

Fix: update ITKExpat_INCLUDE_DIRS in the system-expat branch to
${ITKExpat_BINARY_DIR} where itk_expat.h is now generated.

Also remove any stale src/itk_expat.h left over from builds
predating bfc8e07 so that incremental builds do not pick up the
old copy.

Reported-by: Sean McBride (@seanm) in PR InsightSoftwareConsortium#5838

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hjmjohnson

Modules/ThirdParty/Expat/CMakeLists.txt

@@ -10,7 +10,10 @@ mark_as_advanced(ITK_USE_SYSTEM_EXPAT)
 
 if(ITK_USE_SYSTEM_EXPAT)
   find_package(EXPAT REQUIRED)
-  set(ITKExpat_INCLUDE_DIRS ${ITKExpat_BINARY_DIR}/src)
+  set(
+    ITKExpat_INCLUDE_DIRS
+    ${ITKExpat_BINARY_DIR} # <- for itk_expat.h
+  )
   set(ITKExpat_SYSTEM_INCLUDE_DIRS "${EXPAT_INCLUDE_DIR}")
   set(ITKExpat_LIBRARIES "${EXPAT_LIBRARY}")
   set(ITKExpat_NO_SRC 1)
@@ -44,6 +47,10 @@ else()
 endif()
 
 configure_file(itk_expat.h.in ${ITKExpat_BINARY_DIR}/itk_expat.h)
+# Remove stale itk_expat.h from pre-bfc8e07d build trees that generated it
+# into the src/ subdirectory.  A leftover copy can shadow the current file
+# and confuse incremental builds.
+file(REMOVE ${ITKExpat_BINARY_DIR}/src/itk_expat.h)
 if(NOT ITK_USE_SYSTEM_EXPAT AND NOT ITK_FUTURE_LEGACY_REMOVE)
   # Configure a file with the name <expat.h> as was the default in ITKv5 and earlier
   # so that "#include <expat.h>" finds the itk mangled symbols during linkage stage.