This is a standard environment that doesn't contain a C/C++
compiler. This is mostly to prevent trivial builders like runCommand
and substituteAll from pulling in gcc for simple configuration changes
on NixOS.
* If the environment variable buildCommand is set, then eval that
instead of doing the build phases. This is used by the runCommand
function in all-packages.nix to allow one-lines like
foo = runCommand "foo" {} "mkdir $out; echo foo > $out/foo";
svn path=/nixpkgs/trunk/; revision=7298
