accomodate excessively old gcc versions for raspbian mpy-cross cross-build

Author: jepler

py/sequence.c

@@ -34,6 +34,11 @@
 
 #define SWAP(type, var1, var2) { type t = var2; var2 = var1; var1 = t; }
 
+#if __GNUC__ < 5
+// n.b. does not actually detect overflow!
+#define __builtin_mul_overflow(a, b, x) (*(x) = (a) * (b), false)
+#endif
+
 // Detect when a multiply causes an overflow.
 size_t mp_seq_multiply_len(size_t item_sz, size_t len) {
     size_t new_len;