Fix Libc++ build with MinGW64

Summary: This patch corrects the build errors I encountered when building on MinGW64.

Reviewers: mati865, rnk, compnerd, smeenai, bcraig

Reviewed By: mati865, smeenai

Subscribers: martell, chapuni, cfe-commits

Differential Revision: https://reviews.llvm.org/D33082

llvm-svn: 304360
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 1ec026252b3027f06c8a561d93fc0a3801107bba
diff --git a/include/wchar.h b/include/wchar.h
index 25a318f..a5666e1 100644
--- a/include/wchar.h
+++ b/include/wchar.h
@@ -166,7 +166,7 @@
 }
 #endif
 
-#if defined(__cplusplus) && defined(_LIBCPP_MSVCRT)
+#if defined(__cplusplus) && defined(_LIBCPP_MSVCRT_LIKE)
 extern "C" {
 size_t mbsnrtowcs(wchar_t *__restrict dst, const char **__restrict src,
                   size_t nmc, size_t len, mbstate_t *__restrict ps);