Support more configurability in GN toolchains

tools/gn/example/hello_shared.h

+// 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.
+
+#ifndef HELLO_SHARED_H_
+#define HELLO_SHARED_H_
+
+#if defined(WIN32)
+
+#if defined(HELLO_SHARED_IMPLEMENTATION)
+#define HELLO_EXPORT __declspec(dllexport)
+#define HELLO_EXPORT_PRIVATE __declspec(dllexport)
+#else
+#define HELLO_EXPORT __declspec(dllimport)
+#define HELLO_EXPORT_PRIVATE __declspec(dllimport)
+#endif  // defined(HELLO_SHARED_IMPLEMENTATION)
+
+#else
+
+#if defined(HELLO_IMPLEMENTATION)
+#define HELLO_EXPORT __attribute__((visibility("default")))
+#define HELLO_EXPORT_PRIVATE __attribute__((visibility("default")))
+#else
+#define HELLO_EXPORT
+#define HELLO_EXPORT_PRIVATE
+#endif  // defined(HELLO_IMPLEMENTATION)
+
+#endif
+
+HELLO_EXPORT const char* GetSharedText();
+
+#endif  // HELLO_SHARED_H_