Update m3d.h

Fix review warning.

code/M3D/m3d.h

@@ -87,7 +87,11 @@ typedef uint16_t M3D_INDEX;
 #ifndef _MSC_VER
 #define _inline __inline__
 #define _pack __attribute__((packed))
-#define _unused __attribute__((unused))
+#  ifdef __cplusplus // only for c++ code
+#    define _unused __attribute__((unused))
+#  else // or c code just use void
+#    define _unused (void)
+#  endif
 #else
 #define _inline
 #define _pack