Change glutin headless mode to be a build config, as it breaks some Linux distros linking to both.

The majority of this change is simply re-arranging the code in the glutin port
so that the windowed/headless code is configured at build time rather
than runtime. There shouldn't be any functional difference as a result of this change.

components/servo/Cargo.lock

@@ -335,7 +335,7 @@ dependencies = [
 [[package]]
 name = "glutin"
 version = "0.0.2"
-source = "git+https://github.com/servo/glutin?ref=servo#db27370a1cbafcbfcaeee52a44076a61b3e0573c"
+source = "git+https://github.com/servo/glutin?ref=servo#ec6b4d0fff12ef607db422508ae005ba91406f5b"
 dependencies = [
  "android_glue 0.0.1 (git+https://github.com/servo/android-rs-glue?ref=servo)",
  "cocoa 0.1.1 (git+https://github.com/servo/rust-cocoa)",

components/servo/Cargo.toml

@@ -27,8 +27,10 @@ path = "../../tests/contenttest.rs"
 harness = false
 
 [features]
-default = ["glutin_app"]
+default = ["glutin_app", "window"]
 glfw = ["glfw_app"]
+window = ["glutin_app/window"]
+headless = ["glutin_app/headless"]
 
 [dependencies.compositing]
 path = "../compositing"