New build flag system, convert Google Now flag

build/buildflag.h

+// Copyright 2015 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 BUILD_BUILDFLAG_H_
+#define BUILD_BUILDFLAG_H_
+
+// These macros un-mangle the names of the build flags in a way that looks
+// natural, and gives errors if the flag is not defined. Normally in the
+// preprocessor it's easy to make mistakes that interpret "you haven't done
+// the setup to know what the flag is" as "flag is off". Normally you would
+// include the generated header rather than include this file directly.
+//
+// This is for use with generated headers. See build/build_header.gni.
+
+// This dance of two macros does a concatenation of two preprocessor args using
+// ## doubly indirectly because using ## directly prevents macros in that
+// parameter from being expanded.
+#define BUILDFLAG_CAT_INDIRECT(a, b) a ## b
+#define BUILDFLAG_CAT(a, b) BUILDFLAG_CAT_INDIRECT(a, b)
+
+// Accessor for build flags.
+//
+// To test for a value, if the build file specifies:
+//
+//   ENABLE_FOO=true
+//
+// Then you would check at build-time in source code with:
+//
+//   #include "foo_flags.h"  // The header the build file specified.
+//
+//   #if BUILDFLAG(ENABLE_FOO)
+//     ...
+//   #endif
+//
+// There will no #define called ENABLE_FOO so if you accidentally test for
+// whether that is defined, it will always be negative. You can also use
+// the value in expressions:
+//
+//   const char kSpamServerName[] = BUILDFLAG(SPAM_SERVER_NAME);
+//
+// Because the flag is accessed as a preprocessor macro with (), an error
+// will be thrown if the proper header defining the internal flag value has
+// not been included.
+#define BUILDFLAG(flag) (BUILDFLAG_CAT(BUILDFLAG_INTERNAL_, flag)())
+
+#endif  // BUILD_BUILDFLAG_H_