Use builtin addition overflow checker

This adds wrappers for the __builtin add overflow checkers present
in gcc 5+ and recent clang as well as fallback implementation for
other compilers.

include/mimalloc-internal.h

@@ -109,6 +109,9 @@ bool        _mi_page_is_valid(mi_page_t* page);
 #define mi_likely(x)       (x)
 #endif
 
+#ifndef __has_builtin
+#define __has_builtin(x)  0
+#endif
 
 #if defined(_MSC_VER)
 #define mi_decl_noinline   __declspec(noinline)
@@ -146,6 +149,17 @@ static inline bool mi_mul_overflow(size_t size, size_t count, size_t* total) {
           && size > 0 && (SIZE_MAX / size) < count);
 }
 
+// Overflow detecting addition
+static inline bool mi_add_overflow(size_t a, size_t b, size_t* total) {
+#if __has_builtin(__builtin_add_overflow) || __GNUC__ >= 5
+  return __builtin_add_overflow(a, b, total);
+#else
+   if (a >= (SIZE_MAX - b)) return true; // overflow
+   *total = a + b;
+   return false;
+#endif
+}
+
 // Align a byte size to a size in _machine words_,
 // i.e. byte size == `wsize*sizeof(void*)`.
 static inline size_t _mi_wsize_from_size(size_t size) {