add build system support for partial clobbers

[mozbhearsum]

In the initial Buildbot configs over in https://github.com/brson/servo-buildbot/blob/master/master/master.cfg there's a few steps that would be good to get into the build system behind a target we can call from Buildbot. Specifically:
mv build/obj/src/rust .
rm -rf build/obj
mkdir -p build/obj/src
mv rust build/obj/src

Getting these into the build system will help deal with platform differences when they come up.

[brson]

I'll look at this this week.

[brson]

@bhearsum I added make backup-rust and make restore-rust with the idea that a clobber build would look something like:

cd $build-dir
make backup-rust
rm * -rf
make restore-rust
../configure
make

Is this sufficient?

[mozbhearsum]

I think this will be sufficient, but configure needs to run before restore-rust (otherwise there's no Makefile). I need to test it in the context of Buildbot still, I'll let you know if I have any issues at that point.

[mozbhearsum]

OK, so there's one little bootstrapping issue - the first time we do a build, backup-rust doesn't do anything (because there's no "rust" to backup). Then restore-rust deletes rust, and can't replace it with anything. All of this ends up with a failure early in make because that directory doesn't exist. I think this is fixable by having restore-rust be a no-op if there's no backup.

[metajack]

@bhearsum try this patch

diff --git a/Makefile.in b/Makefile.in
index c1b1ac8..7cda28f 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -275,5 +275,4 @@ backup-rust:
    mv src/rust ..

 restore-rust:
-   rm -rf src/rust
-   mv ../rust src/
+   if [ -d ../rust ]; then rm -rf src/rust; mv ../rust src/; fi

[mozbhearsum]

Yup, that works.

[jdm]

This is in production.