Use _CPPUNWIND instead of _HAS_EXCEPTIONS with MSVC.

_HAS_EXCEPTIONS is specific to the MSVC STL and defining it to 0 causes
problems with libc++, so libc++ users may leave it undefined. This can
cause GTEST_HAS_EXCEPTIONS to be defined incorrectly if the user has
disabled exceptions via the compiler, which can lead to build errors.

_CPPUNWIND is a builtin macro provided by the compiler so it should
work with both STLs.

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

@@ -464,8 +464,11 @@ typedef struct _RTL_CRITICAL_SECTION GTEST_CRITICAL_SECTION;
 #ifndef GTEST_HAS_EXCEPTIONS
 // The user didn't tell us whether exceptions are enabled, so we need
 // to figure it out.
-# if defined(_MSC_VER) || defined(__BORLANDC__)
-// MSVC's and C++Builder's implementations of the STL use the _HAS_EXCEPTIONS
+# if defined(_MSC_VER) && defined(_CPPUNWIND)
+// MSVC defines _CPPUNWIND to 1 iff exceptions are enabled.
+#  define GTEST_HAS_EXCEPTIONS 1
+# elif defined(__BORLANDC__)
+// C++Builder's implementation of the STL uses the _HAS_EXCEPTIONS
 // macro to enable exceptions, so we'll do the same.
 // Assumes that exceptions are enabled by default.
 #  ifndef _HAS_EXCEPTIONS