fix __cplusplus value detection for MSVC (#275)

Author: smithx

CMakeLists.txt

@@ -102,7 +102,7 @@ else ()
         add_definitions(-D_CRT_SECURE_NO_WARNINGS)
     endif ()
     add_definitions(-DBOOST_ALL_NO_LIB)
-    set(MSVC_CXX_FLAGS ${MSVC_CXX_FLAGS} /wd4503 /bigobj)
+    set(MSVC_CXX_FLAGS ${MSVC_CXX_FLAGS} /wd4503 /bigobj /Zc:__cplusplus)
 
     if (CMAKE_BUILD_TYPE MATCHES "Debug" AND JINJA2CPP_MSVC_RUNTIME_TYPE)
         set(JINJA2CPP_MSVC_RUNTIME_TYPE "${JINJA2CPP_MSVC_RUNTIME_TYPE}d")