Make darwin happy.

src/rt/rust_log.cpp

@@ -269,8 +269,9 @@ void update_log_settings(void* crate_map, char* settings) {
                      n_dirs, &n_matches);
 
     if (n_matches < n_dirs) {
-        printf("warning: got %u RUST_LOG specs, enabled %u flags.",
+        printf("warning: got %" PRIdPTR " RUST_LOG specs, "
-               n_dirs, n_matches);
+               "enabled %" PRIdPTR " flags.",
+               (uintptr_t)n_dirs, (uintptr_t)n_matches);
     }
 
     free(buffer);