Disable non-portable compiler extensions.

Author: kan

cmake/SetCXXStandard.cmake

@@ -10,5 +10,5 @@ set(CMAKE_CXX_STANDARD_REQUIRED ON)
 if (NOT "${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
   ### Even though CMAKE_CXX_STANDARD_REQUIRED is supported since CMake 3.1, it doesn't work for Emscripten em++ together with
   ### Cmake 3.6.
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++${CMAKE_CXX_STANDARD}")
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++${CMAKE_CXX_STANDARD}")
 endif()