add --bindir and --sysconfdir support

[Keruspe]

No description provided.

[Keruspe]

This is the first step for adding bindir support to rustbuild.
A concrete example of why it is needed:
On exherbo we install everything that is arch-specific in /usr/<arch>/{bin,lib,libexec} and everything arch-independant to /usr/share and friends.
With the current setup, mostly everything would install fine with rustbuild if we set the prefix to /usr/<arch> for example but some things like cargo's shell completions are wrongly installed inside /usr/<arch>/share.
If this gets accepted I'll send a followup pull request for doing the same with sysconfdir (same problem, bash completion gets installed to /usr/<arch>/etc. (Of course I can add that directly to this pull request, if you want)

[Keruspe]

I figure it's just as easy to review both at the same time so I pushed the sysconfdir stuff too.

[alexcrichton]

Thanks and sorry for the delay!