Don't redefine __WINE_USE_MSVCRT (msvc warning).
Don't return a void value in swab().
'long long' is not recognised by msvc, use __int64.

diff --git a/include/msvcrt/mbstring.h b/include/msvcrt/mbstring.h
index cdbebf7..17f60cd 100644
--- a/include/msvcrt/mbstring.h
+++ b/include/msvcrt/mbstring.h
@@ -19,7 +19,9 @@
  */
 #ifndef __WINE_MBSTRING_H
 #define __WINE_MBSTRING_H
+#ifndef __WINE_USE_MSVCRT
 #define __WINE_USE_MSVCRT
+#endif
 
 #ifndef MSVCRT
 # ifdef USE_MSVCRT_PREFIX