auto merge of #12284 : brson/rust/install, r=alexcrichton

Work toward #9876.

This adds `prepare.mk`, which is simply a more heavily-parameterized `install.mk`, then uses `prepare` to implement both `install` and the windows installer (`dist`). Smoke tested on both Linux and Windows.

src/librustdoc/lib.rs

@@ -12,6 +12,7 @@
 #[desc = "rustdoc, the Rust documentation extractor"];
 #[license = "MIT/ASL2"];
 #[crate_type = "dylib"];
+#[crate_type = "rlib"];
 
 #[feature(globs, struct_variant, managed_boxes)];