ENH: Upgrade ITK from C++11 to C++14

Follow-up to commit babb677
"COMP: Enforce building ITK with C++11", Hans Johnson, 19 January 2018

Author: N-Dekker

CMake/itkCompilerChecks.cmake

@@ -28,13 +28,13 @@ if (CMAKE_CXX_COMPILER_ID STREQUAL "Intel" AND
   message(FATAL_ERROR "Intel C++ (ICC) 14.0 or later is required.")
 endif ()
 
-# Make sure we have C++11 enabled.
-if(NOT ITK_IGNORE_CMAKE_CXX11_CHECKS)
+# Make sure we have C++14 enabled.
+if(NOT ITK_IGNORE_CMAKE_CXX14_CHECKS)
   # Needed to make sure libraries and executables not built by the
-  # itkModuleMacros still have the C++11 compiler flags enabled
+  # itkModuleMacros still have the C++14 compiler flags enabled
   # Wrap this in an escape hatch for unknown compilers
   if(NOT CMAKE_CXX_STANDARD)
-    set(CMAKE_CXX_STANDARD 11) # Supported values are ``11``, ``14``, and ``17``.
+    set(CMAKE_CXX_STANDARD 14) # Supported values are 14, 17, 20, and 23.
   endif()
   if(NOT CMAKE_CXX_STANDARD_REQUIRED)
     set(CMAKE_CXX_STANDARD_REQUIRED ON)

CMakeLists.txt

@@ -35,15 +35,15 @@ foreach(pold "CMP0120") # CMP0120 will require non-trivial effort to address
   endif()
 endforeach()
 
-# ==== Define language standard configurations requiring at least c++11 standard
-if(CMAKE_CXX_STANDARD EQUAL "98" )
-   message(FATAL_ERROR "CMAKE_CXX_STANDARD:STRING=98 is not supported in ITK version 5 and greater.")
+# ==== Define language standard configurations requiring at least c++14 standard
+if(CMAKE_CXX_STANDARD EQUAL "98" OR CMAKE_CXX_STANDARD LESS "14")
+   message(FATAL_ERROR "C++98 to C++11 are no longer supported in ITK version 5.3 and greater.")
 endif()
 
 #####
 ##  Set the default target properties for ITK
 if(NOT CMAKE_CXX_STANDARD)
-  set(CMAKE_CXX_STANDARD 11) # Supported values are ``11``, ``14``, and ``17``.
+  set(CMAKE_CXX_STANDARD 14) # Supported values are 14, 17, 20, and 23.
 endif()
 if(NOT CMAKE_CXX_STANDARD_REQUIRED)
   set(CMAKE_CXX_STANDARD_REQUIRED ON)