refactor: Replace find_package(ITK) with FetchContent integration

- Replace direct find_package(ITK) with CMake/ITKFetchContent.cmake
- FetchContent intelligently finds pre-built ITK via ITK_DIR or downloads
- Fix itk_module_impl condition: use itk-module variable instead of COMMAND
  to properly distinguish between superbuild/module context vs standalone

Author: blowekamp

CMake/ITKFetchContent.cmake

@@ -0,0 +1,60 @@
+#-----------------------------------------------------------------------------
+# Get and build ITK using FetchContent
+
+include(FetchContent)
+
+# Set ITK Git repository and tag
+set(ITK_GIT_REPOSITORY "https://github.com/InsightSoftwareConsortium/ITK.git")
+set(ITK_GIT_TAG "main")
+
+if(NOT DEFINED ITK_BUILD_DEFAULT_MODULES)
+  set(ITK_BUILD_DEFAULT_MODULES ON)
+endif()
+
+# Configure ITK-specific options that need to be set before FetchContent_Declare
+if(NOT ${BUILD_SHARED_LIBS})
+  set(CMAKE_C_VISIBILITY_PRESET "hidden")
+  set(CMAKE_CXX_VISIBILITY_PRESET "hidden")
+  set(ITK_TEMPLATE_VISIBILITY_DEFAULT OFF)
+endif()
+
+set(Modile_ITKReview ON CACHE INTERNAL "")
+set(Module_SphinxExamples OFF CACHE INTERNAL "")
+
+# Set ITK build options
+set(ITK_USE_KWSTYLE OFF)
+set(BUILD_TESTING OFF)
+set(BUILD_EXAMPLES OFF)
+
+FetchContent_Declare(
+  ITK
+  GIT_REPOSITORY "${ITK_GIT_REPOSITORY}"
+  GIT_TAG "${ITK_GIT_TAG}"
+  EXCLUDE_FROM_ALL
+  FIND_PACKAGE_ARGS
+    NAMES
+    ITK
+)
+
+FetchContent_MakeAvailable(ITK)
+
+# Check if FetchContent used find_package() or fetched from source
+FetchContent_GetProperties(ITK)
+if(ITK_SOURCE_DIR)
+  message(STATUS "ITK fetched from repository and built from source")
+  message(STATUS "  Source directory: ${ITK_SOURCE_DIR}")
+  message(STATUS "  Binary directory: ${ITK_BINARY_DIR}")
+  set(ITK_DIR "${ITK_BINARY_DIR}")
+
+  include(${ITK_DIR}/ITKConfig.cmake)
+elseif(DEFINED ITK_FOUND)
+  message(STATUS "ITK found via find_package()")
+  # ITK_DIR should already be set by find_package()
+else()
+  message(FATAL_ERROR "ITK configuration failed - no targets available")
+endif()
+
+# These ITK options conflict with ITKSphinxExamples build options.
+# Allow ITKSphinxExamples' cache variables to be respected.
+unset(BUILD_TESTING)
+unset(BUILD_EXAMPLES)

CMakeLists.txt

@@ -79,8 +79,9 @@ endif()
 #Optional if we are building as an ITK module, these variables will be pre-defined
 if(NOT ITK_VERSION)
   # To get the ITK version information from external package when building
-  # stand alone, ITK is required.
-  find_package(ITK 5.4.2 REQUIRED)
+  # stand alone, ITK is required. ITK can be provided via ITK_DIR or fetched
+  # from source using FetchContent.
+  include(${CMAKE_CURRENT_SOURCE_DIR}/CMake/ITKFetchContent.cmake)
 endif()
 set(ITKSphinxExamples_VERSION_MAJOR ${ITK_VERSION_MAJOR})
 set(ITKSphinxExamples_VERSION_MINOR ${ITK_VERSION_MINOR})
@@ -308,7 +309,8 @@ configure_file("${ITKSphinxExamples_SOURCE_DIR}/Utilities/CreateNewExample.py.in
 
 # Build the example executables?
 option(BUILD_EXECUTABLES "Build the example executables." ON)
-if(COMMAND itk_module_impl)
+# Use itk-module (set by ITK's module system) to detect if we are building as an ITK module or stand alone.
+if(itk-module)
   itk_module_impl()
 else()
   if(BUILD_EXECUTABLES)