Limit libstdc++ workaround to not upset libc++ with pragma visibility.

diff --git a/intl/icu/source/common/unicode/std_string.h b/intl/icu/source/common/unicode/std_string.h
--- a/intl/icu/source/common/unicode/std_string.h
+++ b/intl/icu/source/common/unicode/std_string.h
@@ -22,16 +22,16 @@
  * \brief C++ API: Central ICU header for including the C++ standard <string>
  *                 header and for related definitions.
  */
 
 #include "unicode/utypes.h"
 
 #if U_HAVE_STD_STRING
 
-#if !defined(_MSC_VER)
+#if defined(__GLIBCXX__)
 namespace std { class type_info; } // WORKAROUND: http://llvm.org/bugs/show_bug.cgi?id=13364
 #endif
 #include <string>
 
 #endif  // U_HAVE_STD_STRING
 
 #endif  // __STD_STRING_H__