Auto merge of #2134 - alexcrichton:build-script-input, r=brson

Currently Cargo is quite conservative in how it determines whether a build
script should be run. The heuristic used is "did any file in the project
directory change", but this is almost always guaranteed to be too coarse
grained in situations like:

* If the build script takes a long time to run it's advantageous to run it as
  few times as possible. Being able to inform Cargo about precisely when a build
  script should be run should provide more robust support here.
* Build scripts may not always have all of their dependencies in-tree or in the
  crate root. Sometimes a dependency could be elsewhere in a repository and
  scripts need a method of informing Cargo about this (as currently these
  compiles don't happen then they should).

This commit adds this support in build scripts via a new `rerun-if-changed`
directive which can be printed to standard output (using the standard Cargo
metadata format). The value for this key is a path relative to the crate root,
and Cargo will only look at these paths when determining whether to rerun the
build script. Any other file changes will not trigger the build script to be
rerun.

Currently the printed paths may either be a file or a directory, and a directory
is deeply traversed. The heuristic for trigger a rerun is detecting whether any
input file has been modified since the build script was last run (determined by
looking at the modification time of the output file of the build script). This
current implementation means that if you depend on a directory and then delete a
file within it the build script won't be rerun, but this is already the case and
can perhaps be patched up later.

Future extensions could possibly include the usage of glob patterns in build
script paths like the `include` and `exclude` features of `Cargo.toml`, but
these should be backwards compatible to add in the future.

Closes #1162

diff --cc src/cargo/ops/cargo_compile.rs
Simple merge

diff --cc src/cargo/ops/cargo_rustc/context.rs
Simple merge

diff --cc src/cargo/ops/cargo_rustc/custom_build.rs
Simple merge

diff --cc src/cargo/ops/cargo_rustc/fingerprint.rs
Simple merge

diff --cc src/doc/build-script.md
Simple merge