Use timingsafe_memcmp() where available.

See ticket 17944; patch from "logan".

changes/17944

@@ -0,0 +1,3 @@
+  o Minor features (portability):
+    - Use timingsafe_memcmp() where available. Closes ticket 17944;
+      patch from "logan".

configure.ac

@@ -381,6 +381,7 @@ AC_CHECK_FUNCS(
         backtrace_symbols_fd \
         clock_gettime \
 	eventfd \
+	timingsafe_memcmp \
         flock \
         ftime \
         getaddrinfo \

src/common/di_ops.c

@@ -25,6 +25,9 @@
 int
 tor_memcmp(const void *a, const void *b, size_t len)
 {
+#ifdef HAVE_TIMINGSAFE_MEMCMP
+  return timingsafe_memcmp(a, b, len);
+#else
   const uint8_t *x = a;
   const uint8_t *y = b;
   size_t i = len;
@@ -83,6 +86,7 @@ tor_memcmp(const void *a, const void *b, size_t len)
   }
 
   return retval;
+#endif /* timingsafe_memcmp */
 }
 
 /**