Index: libcxx/trunk/include/__config
===================================================================
--- libcxx/trunk/include/__config
+++ libcxx/trunk/include/__config
@@ -229,8 +229,9 @@
 #  define _LIBCPP_SHORT_WCHAR   1
 // Both MinGW and native MSVC provide a "MSVC"-like enviroment
 #  define _LIBCPP_MSVCRT_LIKE
-// If mingw not explicitly detected, assume using MS C runtime only.
-#  ifndef __MINGW32__
+// If mingw not explicitly detected, assume using MS C runtime only if
+// a MS compatibility version is specified.
+#  if defined(_MSC_VER) && !defined(__MINGW32__)
 #    define _LIBCPP_MSVCRT // Using Microsoft's C Runtime library
 #  endif
 #  if (defined(_M_AMD64) || defined(__x86_64__)) || (defined(_M_ARM) || defined(__arm__))