Added info on the build system to contributing guide

I recently wrote a blog post on contributing to the Rust compiler which
gained some interest. It was mentioned in a comment on Reddit that it
would be useful to integrate some of the information from that post to
the official contributing guide.

This is the start of my efforts to integrate what I wrote with the
official guide.

This commit adds information on the build system. It is not a complete
guide on the build system, but it should be enough to provide a good
starting place for those wishing to contribute.

CONTRIBUTING.md

@@ -6,6 +6,7 @@ links to the major sections:
 
 * [Feature Requests](#feature-requests)
 * [Bug Reports](#bug-reports)
+* [The Build System](#the-build-system)
 * [Pull Requests](#pull-requests)
 * [Writing Documentation](#writing-documentation)
 * [Issue Triage](#issue-triage)
@@ -77,6 +78,60 @@ to do this is to invoke `rustc` like this:
 $ RUST_BACKTRACE=1 rustc ...
 ```
 
+## The Build System
+
+The build system for Rust is complex. It covers bootstrapping the compiler,
+running tests, building documentation and more. Unless you are familiar with
+Makefiles, I wouldn't suggest trying to understand everything going on in
+Rust's setup - there's a lot there, and you can get lost trying to understand
+it all.
+
+If Makefiles are your thing, though, all the configuration lives in
+[the `mk` directory][mkdir] in the project root.
+
+[mkdir]: https://github.com/rust-lang/rust/tree/master/mk/
+
+### Configuration
+
+Before you can start building the compiler you need to configure the build for
+your system. In most cases, that will just mean using the defaults provided
+for Rust. Configuring involves invoking the `configure` script in the project
+root.
+
+```
+./configure
+```
+
+There are large number of options accepted by this script to alter the
+configuration used later in the build process. Some options to note:
+
+- `--enable-debug` - Build a debug version of the compiler (disables optimizations)
+- `--disable-valgrind-rpass` - Don't run tests with valgrind
+- `--enable-clang` - Prefer clang to gcc for building dependencies (ie LLVM)
+- `--enable-ccache` - Invoke clang/gcc with ccache to re-use object files between builds
+
+To see a full list of options, run `./configure --help`.
+
+### Useful Targets
+
+Some common make targets are:
+
+- `make rustc-stage1` - build up to (and including) the first stage. For most
+  cases we don't need to build the stage2 compiler, so we can save time by not
+  building it. The stage1 compiler is a fully functioning compiler and
+  (probably) will be enough to determine if your change works as expected.
+- `make check` - build the full compiler & run all tests (takes a while). This
+  is what gets run by the continuous integration system against your pull
+  request. You should run this before submitting to make sure your tests pass
+  & everything builds in the correct manner.
+- `make check-stage1-std NO_REBUILD=1` - test the standard library without
+  rebuilding the entire compiler
+- `make check TESTNAME=<path-to-test-file>.rs` - Run a single test file
+- `make check-stage1-rpass TESTNAME=<path-to-test-file>.rs` - Run a single
+  rpass test with the stage1 compiler (this will be quicker than running the
+  command above as we only build the stage1 compiler, not the entire thing).
+  You can also leave off the `-rpass` to run all stage1 test types.
+
 ## Pull Requests
 
 Pull requests are the primary mechanism we use to change Rust. GitHub itself