[Nix-dev] Parallel building support, 2nd attempt

[Peter Simons]

Hi,

support for parallel builds is now in stdenv-updates. The only major change
relative to the diff I posted is that the generic builder can auto-detect the
number of available cores. To enable that feature, "build-cores" must be set to
"0", "guess" or "auto". The same applies to the "--cores" command line argument
(and, yes, it does work for nix-store, nix-build, and nix-env).

I tested the patch successfully on Linux/x86, Linux/x86_64, and Darwin/x86.
However, if you experience any problems, please let me know.

Eelco, I didn't want to bump the nixUnstable expression to the latest version,
so I applied the changes as a patch. I committed my modifications to the Nix
repository, too, though. I'll add documentation to the XML files ASAP.

Take care,
Peter