fwd: Update __MCF_MAY_THROW to use __MCF_CXX()

(cherry picked from commit 140ef0e)
Signed-off-by: LIU Hao <lh_mouse@126.com>

mcfgthread/fwd.h

@@ -56,8 +56,13 @@
 #  define __MCF_UNREACHABLE   __assume(0)
 #endif
 
-#if defined _MSC_VER && defined __cplusplus
-#  define __MCF_MAY_THROW   throw(...)
+/* Generally speaking, functions are either `__MCF_MAY_THROW` or `noexcept`. This
+ * macro is necessary for Visual Studio (but not CL.EXE from command-line), which
+ * defaults to `/EHsc`, which assumes that `extern "C"` functions can't throw C++
+ * exceptions. Not only is this behavior not conforming to the C++ standard, it
+ * can also result in wrong code about `__MCF_gthr_call_once_seh()`.  */
+#if defined _MSC_VER
+#  define __MCF_MAY_THROW   __MCF_CXX(throw(...))
 #else
 #  define __MCF_MAY_THROW
 #endif