Do not fail with _Alignas(0) and _Alignas(1), used by autotools

Used by configure scripts, generated by newer GNU autotools
Autotools now checks for C11 and C99 compatibbility

--
bye bye Detlef

tccgen.c

@@ -4477,7 +4477,7 @@ static int parse_btype(CType *type, AttributeDef *ad)
                     type_size(&type1, &n);
               } else {
                   n = expr_const();
-                  if (n <= 0 || (n & (n - 1)) != 0)
+                  if (n < 0 || ((n > 1) & ((n & (n - 1)) != 0)))
                     tcc_error("alignment must be a positive power of two");
               }
               skip(')');