Features: Run GNU feature tests with std=c++14 when available.

Modules/Compiler/GNU-CXX.cmake

@@ -24,7 +24,10 @@ elseif(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.3)
   set(CMAKE_CXX11_EXTENSION_COMPILE_OPTION "-std=gnu++0x")
 endif()
 
-if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.8)
+if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.9)
+  set(CMAKE_CXX14_STANDARD_COMPILE_OPTION "-std=c++14")
+  set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++14")
+elseif (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.8)
   set(CMAKE_CXX14_STANDARD_COMPILE_OPTION "-std=c++1y")
   set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++1y")
 endif()