COMP: Initialize CXX standard in ITK_USE_FILE

Code that builds against ITK may not be built with a sufficient or
compatible C++ standard.

Use ITKInitializeCXXStandard.cmake in client code, which calls

  include(${ITK_USE_FILE})

according to our existing documentation, which will set
CMAKE_CXX_STANDARD and CMAKE_CXX_STANDARD_REQUIRED if not already set.

To address build errors as discussed in:

  https://discourse.itk.org/t/hello-world-from-itk-examples-installation-not-compiling-for-itk-5-3/4441/8

CMake/UseITK.cmake

@@ -22,6 +22,7 @@ set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${ITK_REQUIRED_CXX_FLAGS}")
 set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${ITK_REQUIRED_LINK_FLAGS}")
 set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} ${ITK_REQUIRED_LINK_FLAGS}")
 set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} ${ITK_REQUIRED_LINK_FLAGS}")
+include(${ITK_CMAKE_DIR}/ITKInitializeCXXStandard.cmake)
 
 # Add include directories needed to use ITK.
 include_directories(BEFORE ${ITK_INCLUDE_DIRS})