Work on GN toolchain definitions and build

build/toolchain/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.
 
+# TODO(brettw) Use "gcc_toolchain.gni" like the Linux toolchains. This requires
+# some enhancements since the commands on Mac are slightly different than on
+# Linux.
+
 # Should only be running on Mac.
 assert(is_mac || is_ios)
 
-cc = rebase_path("//third_party/llvm-build/Release+Asserts/bin/clang", ".", root_build_dir)
-cxx = rebase_path("//third_party/llvm-build/Release+Asserts/bin/clang++", ".", root_build_dir)
+# Clang variables.
+import("../clang.gni")
+
+if (is_clang) {
+  cc = rebase_path("//third_party/llvm-build/Release+Asserts/bin/clang", ".", root_build_dir)
+  cxx = rebase_path("//third_party/llvm-build/Release+Asserts/bin/clang++", ".", root_build_dir)
+
+  # Set the GYP header for all toolchains when running under Clang.
+  if (is_gyp) {
+    gyp_header = make_clang_global_settings
+  }
+} else {
+  cc = "gcc"
+  cxx = "g++"
+}
 ld = cxx
 
 # This will copy the gyp-mac-tool to the build directory. We pass in the source
 # file of the win tool.
 gyp_mac_tool_source =
   rebase_path("//tools/gyp/pylib/gyp/mac_tool.py", ".", root_build_dir)
 exec_script("setup_toolchain.py", [ gyp_mac_tool_source ], "value")
 
 toolchain("clang") {
   # Make these apply to all tools below.
@@ skipping 53 matching lines @@
   #}
   tool("stamp") {
     command = "\${postbuilds}touch \$out"
     description = "STAMP \$out"
   }
   tool("copy") {
     command = "ln -f \$in \$out 2>/dev/null || (rm -rf \$out && cp -af \$in \$out)"
     description = "COPY \$in \$out"
   }
 }