[algs] Use __builtin_mul_overflow

Compiles to smaller binary.

src/hb-algs.hh

diff --git a/src/hb-algs.hh b/src/hb-algs.hh
index bcd7ad0..9659d63 100644
--- a/src/hb-algs.hh
+++ b/src/hb-algs.hh
@@ -853,10 +853,14 @@
  * Overflow checking.
  */
 
-/* Consider __builtin_mul_overflow use here also */
 static inline bool
 hb_unsigned_mul_overflows (unsigned int count, unsigned int size)
 {
+#if (defined(__GNUC__) && (__GNUC__ >= 4)) || defined(__clang__)
+  unsigned result;
+  return __builtin_mul_overflow (count, size, &result);
+#endif
+
   return (size > 0) && (count >= ((unsigned int) -1) / size);
 }