Only add -lrt on Linux, not on Mac

runtime/vm/BUILD.gn

 # Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 config("libdart_vm_config") {
   libs = [
     "dl",
   ]
 
   if (!is_android) {
-    libs += [
-      "pthread",
-      "rt",
-    ]
+    libs += [ "pthread" ]
+
+    if (is_linux) {
+      libs += [ "rt" ]
+    }
   }
 }
 
 
 static_library("libdart_platform") {
   configs += ["..:dart_config"]
   public_configs = [":libdart_vm_config"]
 
   platform_headers_gypi =
       exec_script("../../tools/gypi_to_gn.py",
@@ skipping 173 matching lines @@
   output = "$target_gen_dir/service_gen.cc"
   outputs = [ output, ]
 
   args = [
     "--output", rebase_path(output, root_build_dir),
     "--outer_namespace", "dart",
     "--table_name", "service",
     "--root_prefix", rebase_path("service/", root_build_dir)] +
     rebase_path(sources, root_build_dir)
 }