Add note re ASSERT_STATIC_EXPR_ZERO()

src/hb-private.hh

@@ -193,6 +193,8 @@ static inline unsigned int ARRAY_LENGTH (const Type (&)[n]) { return n; }
 #define _ASSERT_STATIC0(_line, _cond)	_ASSERT_STATIC1 (_line, (_cond))
 #define ASSERT_STATIC(_cond)		_ASSERT_STATIC0 (__LINE__, (_cond))
 
+/* Note: C++ allows sizeof() of variable-lengh arrays.  So, if _cond is not
+ * constant, it still compiles (ouch!), but at least we'll get a -Wvla warning. */
 #define ASSERT_STATIC_EXPR_ZERO(_cond) (0 * sizeof (char[(_cond) ? 1 : -1]))
 
 #define _PASTE1(a,b) a##b