add ccache support to gn builds

build/toolchain/ccache.gni

+# Copyright (c) 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.
+
+# Defines the configuration of ccache.

[brettw, 2014/11/26 21:18:14]

Can you mention here what ccache is?

[Mostyn Bramley-Moore, 2014/11/26 22:09:52]

Done.

+#
+# TIPS:
+#
+# Set clang_use_chrome_plugins=false since ccache doesn't know how to handle
+# clang plugins.

[Nico, 2014/11/26 17:01:06]

Is there a bug filed with ccache for this?

[Mostyn Bramley-Moore, 2014/11/26 22:09:52]

ccache 3.1.10 onwards supports the -Xclang flag:
https://ccache.samba.org/releasenotes.html#_ccache_3_1_10

Though I've been trying to reproduce my earlier issue on 3.1.9 without luck. 
I'll poke around some more and adjust this comment.

[Mostyn Bramley-Moore, 2014/11/26 22:23:24]

(Turns out I didn't remove the new ccache from my PATH in my test script.)

+#
+# Use ccache 3.2 or later to avoid clang unused argument warnings:
+# https://bugzilla.samba.org/show_bug.cgi?id=8118
+#
+# To avoid -Wparentheses-equality clang warnings, at some cost in terms of
+# speed, you can do:
+# export CCACHE_CPP2=yes
+
+declare_args() {
+  # Set to true to enable ccache.  Probably doesn't work on windows.
+  use_ccache = false
+}