rpath binaries so no LD_LIBRARY_PATH is needed

[graydon]

use of rpath and $ORIGIN when linking, and we won't need LD_LIBRARY_PATH settings at all. implement this.

[eholk]

I started working on this a little, but I will probably switch back to concurrency stuff before long. Here's a patch with my work so far:

https://gist.github.com/1031941

It depends on the getcwd and other functions that I added to the library, which will probably land after I make sure they work on all three platforms.

[graydon]

It's possible this will only "really" work on linux and osx, and we'll have to ship a .bat file for windows that adds our bin and lib directories to %PATH%. That seems like "the normal way" to do toolchains on windows. I think.

[graydon]

As part of this, I'd like to make the stageN directories look as much as possibly like the structure a normal-unix distro would want landing in /usr. That is: bin/rustc, lib/libfoo.so for the runtime libs, and lib/rustc/$arch/libfoo.so for the target libs. I have started fiddling the makefiles and configury to build this way, but it is incomplete. Best bet I think is to build everything redundantly in the old-and-new locations for a bit first, then snapshot and switch over to only using the new locations, then snapshot again and start purging the old locations.

[graydon]

See also #474

[brson]

Done.

[brson]

Reopening because it doesn't seem to be working on mac yet