Added explicit MSGPACK_USE_CPP03 on MSVC2015 and leter.

Author: redboltz

CMakeLists.txt

@@ -41,6 +41,16 @@ IF (MSGPACK_CXX11)
           MESSAGE ( FATAL_ERROR "MSVC doesn't support C++11.")
       ENDIF ()
    ENDIF ()
+ELSE ()
+   IF ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
+      SET (CMAKE_CXX_FLAGS "-std=c++03 ${CMAKE_CXX_FLAGS}")
+   ELSEIF ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
+      SET (CMAKE_CXX_FLAGS "-std=c++03 ${CMAKE_CXX_FLAGS}")
+   ELSEIF ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
+      IF (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 18)
+          SET (CMAKE_CXX_FLAGS "-DMSGPACK_USE_CPP03 ${CMAKE_CXX_FLAGS}")
+      ENDIF ()
+   ENDIF ()
 ENDIF ()
 
 IF (MSGPACK_32BIT)