Add CCACHE infra and turn it on in travis

.travis.yml

@@ -1,5 +1,8 @@
 language: python
 
+env:
+  - CCACHE=/usr/bin/ccache
+
 matrix:
   fast_finish: true
   include:
@@ -13,6 +16,7 @@ matrix:
         directories:
           - .cargo
           - .servo
+          - $HOME/.ccache
       addons:
         apt:
           packages:
@@ -22,6 +26,7 @@ matrix:
             - libosmesa6-dev
             - python-virtualenv
             - xorg-dev
+            - ccache
 
 branches:
   only:

python/servo/build_commands.py

@@ -232,6 +232,9 @@ class MachCommands(CommandBase):
             env['OPENSSL_INCLUDE_DIR'] = path.join(openssl_dir, "include")
             env['OPENSSL_STATIC'] = 'TRUE'
 
+        if not (self.config["build"]["ccache"] == ""):
+            env['CCACHE'] = self.config["build"]["ccache"]
+
         status = call(
             ["cargo", "build"] + opts,
             env=env, cwd=self.servo_crate(), verbose=verbose)

python/servo/command_base.py

@@ -107,6 +107,7 @@ class CommandBase(object):
         self.config["build"].setdefault("android", False)
         self.config["build"].setdefault("mode", "")
         self.config["build"].setdefault("debug-mozjs", False)
+        self.config["build"].setdefault("ccache", "")
 
         self.config.setdefault("android", {})
         self.config["android"].setdefault("sdk", "")

servobuild.example

@@ -36,6 +36,8 @@ rustc-with-gold = true
 android = false
 # Set "debug-mozjs" or use `mach build --debug-mozjs` to build a debug spidermonkey.
 debug-mozjs = false
+# Set to the path to your ccache binary to enable caching of compiler outputs
+#ccache = "/usr/local/bin/ccache"
 
 # Android information
 [android]