Workaround VS bug w.r.t empty arguments to macros

Empty arguments can be passed to macros per C99 and C++11 specs, which
can then be forwarded to other macros.

Visual Studio's compiler has a bug in the following situation:

    #define A(x) #x
    #define B(x, y) A(x)
    B(, b)

In the above case, Visual Studio first expands x to nothing, then
complains that A is not invoked with the right amount of arguments.
However, x in A(x) is still one argument, even if it expands to no
preprocessing tokens.  See also https://stackoverflow.com/a/7674214.

googletest/include/gtest/internal/gtest-internal.h

@@ -79,7 +79,16 @@
 #define GTEST_CONCAT_TOKEN_IMPL_(foo, bar) foo ## bar
 
 // Stringifies its argument.
-#define GTEST_STRINGIFY_(name) #name
+// Work around a bug in visual studio which doesn't accept code like this:
+//
+//   #define GTEST_STRINGIFY_(name) #name
+//   #define MACRO(a, b, c) ... GTEST_STRINGIFY_(a) ...
+//   MACRO(, x, y)
+//
+// Complaining about the argument to GTEST_STRINGIFY_ being empty.
+// This is allowed by the spec.
+#define GTEST_STRINGIFY_HELPER_(name, ...) #name
+#define GTEST_STRINGIFY_(...) GTEST_STRINGIFY_HELPER_(__VA_ARGS__,)
 
 namespace proto2 { class Message; }