Option configuration to enable incremental compilation.

https://internals.rust-lang.org/t/incremental-compilation-beta/4721

servobuild.example

@@ -38,15 +38,22 @@ rustc-with-gold = true
 # or Set "mode = release" or use `mach build --release` for optimized build.
 # Defaults to prompting before building
 #mode = "dev"
+
 # Set "android = true" or use `mach build --android` to build the Android app.
 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"
+
 # Any optional flags that will be added to $RUSTFLAGS
 #rustflags = ""
 
+# Enable rustc’s incremental compilation
+#incremental = false
+
 # Android information
 [android]
 # Defaults to the value of $ANDROID_SDK, $ANDROID_NDK, $ANDROID_TOOLCHAIN, $ANDROID_PLATFORM respectively