Avoid using _vsnprintf on MinGW.

We went to the trouble of getting standard C99 format strings with
__USE_MINGW_ANSI_STDIO, but then used _vsnprintf, which bypasses them
and implements the non-standard Microsoft format strings, leading to
incorrect behaviour.
Ubuntu 14.04's mingw-w64 at least defines a vsnprintf() in all
circumstances, so we should use it.

utils.c

@@ -308,7 +308,10 @@ int string_length_for_printf(size_t s)
 #endif
 
 /* Also lack of vsnprintf before VS2015 */
-#if defined _WINDOWS && !defined __WINE__ && _MSC_VER < 1900
+#if defined _WINDOWS && \
+    !defined __MINGW32__ && \
+    !defined __WINE__ && \
+    _MSC_VER < 1900
 #define vsnprintf _vsnprintf
 #endif