Enable GN build on Mac.

build/config/mac/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/sysroot.gni")
 
 config("sdk") {
   common_flags = [
     "-stdlib=libc++",
   ]
 
   cflags = common_flags
   ldflags = common_flags
 }
 
 # On Mac, this is used for everything except static libraries.
 config("mac_dynamic_flags") {
   ldflags = [
     "-Wl,-search_paths_first",
     "-L.",
 
+    # Forces shared libraries to search for undefined symbols.
+    "-undefined",  "dynamic_lookup",
+
     # Path for loading shared libraries for unbundled binaries.
     "-Wl,-rpath,@loader_path/.",
 
     # Path for loading shared libraries for bundled binaries. Get back from
     # Binary.app/Contents/MacOS.
     "-Wl,-rpath,@loader_path/../../..",
   ]
 }
 
 # On Mac, this is used only for executables.
 config("mac_executable_flags") {
   ldflags = [ "-Wl,-pie" ]  # Position independent.
 }