Support more configurability in GN toolchains

tools/gn/example/BUILD.gn

Index: tools/gn/example/BUILD.gn
diff --git a/tools/gn/example/BUILD.gn b/tools/gn/example/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..2cb013686d761bfba38679e9b24dad3df7861acd
--- /dev/null
+++ b/tools/gn/example/BUILD.gn
@@ -0,0 +1,28 @@
+# Copyright 2014 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.
+
+executable("hello") {
+  sources = [ "hello.cc" ]
+
+  deps = [
+    ":hello_static",
+    ":hello_shared",
+  ]
+}
+
+shared_library("hello_shared") {
+  sources = [
+    "hello_shared.cc",
+    "hello_shared.h"
+  ]
+
+  defines = [ "HELLO_SHARED_IMPLEMENTATION" ]
+}
+
+static_library("hello_static") {
+  sources = [
+    "hello_static.cc",
+    "hello_static.h",
+  ]
+}