Fix compilation error with Mingw32

Fix compilation error on Mingw32 when `_TRUNCATE` is defined. Use
`_TRUNCATE` only if `__MINGW32__` not defined. Fix suggested by
Thomas Glanzmann and Nick Wilson on issue #355
diff --git a/library/platform.c b/library/platform.c
index af3b2f1..68506f5 100644
--- a/library/platform.c
+++ b/library/platform.c
@@ -74,7 +74,7 @@
         return( -1 );
 
     va_start( argp, fmt );
-#if defined(_TRUNCATE)
+#if defined(_TRUNCATE) && !defined(__MINGW32__)
     ret = _vsnprintf_s( s, n, _TRUNCATE, fmt, argp );
 #else
     ret = _vsnprintf( s, n, fmt, argp );