COMP: Bump Validated CMake policies version to 3.29.0

Fix CLANG_FORMAT_EXECUTABLE location with find_program to address:

CMake Error at CMake/ITKClangFormatSetup.cmake:62 (message):
  Missing suitable clang-format executable, set CLANG_FORMAT_EXECUTABLE
  variable to version 8.0
Call Stack (most recent call first):
  CMakeLists.txt:586 (include)

Improve host system detection for the clang-format binary download.

CMake/ITKClangFormatSetup.cmake

@@ -11,7 +11,9 @@
 option(ITK_USE_CLANG_FORMAT "Enable the use of clang-format enforce ITK coding style." ${BUILD_TESTING})
 mark_as_advanced(ITK_USE_CLANG_FORMAT)
 
-if(ITK_USE_CLANG_FORMAT AND NOT EXISTS "${CLANG_FORMAT_EXECUTABLE}")
+find_program(CLANG_FORMAT_EXECUTABLE NAMES clang-format-8.0 clang-format8 clang-format)
+mark_as_advanced(CLANG_FORMAT_EXECUTABLE)
+if(ITK_USE_CLANG_FORMAT AND NOT CLANG_FORMAT_EXECUTABLE)
   # Download pre-built binaries (about 2M) of clang-format extracted from
   # https://releases.llvm.org/download.html and cached on data.kitware.com
   #
@@ -31,14 +33,14 @@ if(ITK_USE_CLANG_FORMAT AND NOT EXISTS "${CLANG_FORMAT_EXECUTABLE}")
   # Android              - Android NDK (GCC, Clang)
   # CrayLinuxEnvironment - Cray supercomputers (Cray compiler)
   # MSYS                 - Windows (MSYS2 shell native GCC)
-  if("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
+  if(CMAKE_HOST_SYSTEM_NAME STREQUAL "Darwin")
     set(CLANG_FORMAT_DOWNLOAD_URL "https://data.kitware.com/api/v1/file/5d274e88877dfcc902effc47/download")
-  elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Windows")
+  elseif(CMAKE_HOST_SYSTEM_NAME STREQUAL "Windows")
     set(CLANG_FORMAT_DOWNLOAD_URL "https://data.kitware.com/api/v1/file/5d2b8775877dfcc902fd8236/download")
-  elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "Linux")
+  elseif (CMAKE_HOST_SYSTEM_NAME MATCHES "Linux" AND CMAKE_HOST_SYSTEM_PROCESSOR STREQUAL "x86_64")
     set(CLANG_FORMAT_DOWNLOAD_URL "https://data.kitware.com/api/v1/file/5d2b8c87877dfcc902fda594/download")
   else()
-    message(FATAL_ERROR "UNKNOWN SYSTEM: Set CLANG_FORMAT_EXECUTABLE manually to the clang-format version 8.0 binary")
+    message(FATAL_ERROR "System binaries not available: set CLANG_FORMAT_EXECUTABLE to clang-format 8 or disable ITK_USE_CLANG_FORMAT.")
   endif()
   file(MAKE_DIRECTORY "${ITK_BINARY_DIR}/temp")
   if(WIN32)
@@ -54,11 +56,6 @@ if(ITK_USE_CLANG_FORMAT AND NOT EXISTS "${CLANG_FORMAT_EXECUTABLE}")
   set(CLANG_FORMAT_EXECUTABLE
       "${ITK_BINARY_DIR}/${CLANG_FORMAT_EXECUTABLE_NAME}"
       CACHE FILEPATH "The binary for clang-format" FORCE)
-endif()
-if(CLANG_FORMAT_EXECUTABLE)
-  mark_as_advanced(CLANG_FORMAT_EXECUTABLE)
-endif()
-if(ITK_USE_CLANG_FORMAT AND NOT EXISTS "${CLANG_FORMAT_EXECUTABLE}")
-  message(FATAL_ERROR "Missing suitable clang-format executable, set CLANG_FORMAT_EXECUTABLE variable to version 8.0")
-  unset(CLANG_FORMAT_EXECUTABLE)
+elseif(ITK_USE_CLANG_FORMAT AND NOT CLANG_FORMAT_EXECUTABLE)
+  message(FATAL_ERROR "Missing suitable clang-format executable, set CLANG_FORMAT_EXECUTABLE to clang-format 8 or disable ITK_USE_CLANG_FORMAT.")
 endif()

CMakeLists.txt

@@ -11,15 +11,15 @@
 # continue to generate policy warnings "CMake Warning (dev)...Policy CMP0XXX is not set:"
 #
 set(ITK_OLDEST_VALIDATED_POLICIES_VERSION "3.16.3")
-set(ITK_NEWEST_VALIDATED_POLICIES_VERSION "3.19.7")
+set(ITK_NEWEST_VALIDATED_POLICIES_VERSION "3.29.0")
 cmake_minimum_required(VERSION ${ITK_OLDEST_VALIDATED_POLICIES_VERSION}...${ITK_NEWEST_VALIDATED_POLICIES_VERSION}
                        FATAL_ERROR)
 
 #
 # Now enumerate specific policies newer than ITK_NEWEST_VALIDATED_POLICIES_VERSION
 # that may need to be individually set to NEW/OLD
 #
-foreach(pnew "CMP0120" "CMP0135")
+foreach(pnew )
   if(POLICY ${pnew})
     cmake_policy(SET ${pnew} NEW)
   endif()