Support more configurability in GN toolchains

tools/gn/example/build/BUILD.gn

Index: tools/gn/example/build/BUILD.gn
diff --git a/tools/gn/example/build/BUILD.gn b/tools/gn/example/build/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..62243725a569f287d94ba65fffc1b5403d456fe3
--- /dev/null
+++ b/tools/gn/example/build/BUILD.gn
@@ -0,0 +1,19 @@
+# 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.
+
+config("compiler_defaults") {
+  if (os == "linux") {
+    cflags = [
+      "-fPIC",
+      "-pthread",
+    ]
+  }
+}
+
+config("executable_ldconfig") {
+  ldflags = [
+    "-Wl,-rpath=\$ORIGIN/",
+    "-Wl,-rpath-link=",
+  ]
+}