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/debug.c b/library/debug.c
index f9229b3..db3924a 100644
--- a/library/debug.c
+++ b/library/debug.c
@@ -91,7 +91,7 @@
 
     va_start( argp, format );
 #if defined(_WIN32)
-#if defined(_TRUNCATE)
+#if defined(_TRUNCATE) && !defined(__MINGW32__)
     ret = _vsnprintf_s( str, DEBUG_BUF_SIZE, _TRUNCATE, format, argp );
 #else
     ret = _vsnprintf( str, DEBUG_BUF_SIZE, format, argp );