XL: Add C++14 language level flags

Help/release/dev/xl-supports-cxx14.rst

+xlc-supports-cxx14
+----------------------
+
+* IBM AIX XL compiler have learned how to compile C++14.

Modules/Compiler/XL-CXX.cmake

@@ -18,6 +18,11 @@ if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 10.1)
   set(CMAKE_CXX11_EXTENSION_COMPILE_OPTION "-qlanglvl=extended0x")
   set(CMAKE_CXX98_STANDARD__HAS_FULL_SUPPORT ON)
   set(CMAKE_CXX11_STANDARD__HAS_FULL_SUPPORT ON)
+  if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 16.1.0)
+    set(CMAKE_CXX14_STANDARD_COMPILE_OPTION "-qlanglvl=extended1y")
+    set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-qlanglvl=extended1y")
+    set(CMAKE_CXX14_STANDARD__HAS_FULL_SUPPORT ON)
+  endif()
 endif ()
 
 __compiler_check_default_language_standard(CXX 10.1 98)

Modules/Compiler/XLClang-CXX.cmake

@@ -15,6 +15,10 @@ if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 13.1.1)
     set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++1y")
     set(CMAKE_CXX14_STANDARD__HAS_FULL_SUPPORT ON)
   endif ()
+  if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 16.1.0)
+    set(CMAKE_CXX14_STANDARD_COMPILE_OPTION  "-std=c++14")
+    set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++14")
+  endif()
 endif()
 
 __compiler_check_default_language_standard(CXX 13.1.1 98)