* guix-build.in (derivations-from-package-expressions): New `system'
parameter. Pass it to `package-derivation'.
(%default-options): Add `system' pair.
(show-help): Describe `--system'.
(%options): Add it.
(guix-build): Check the `system' pair in OPTS; pass it to
`derivations-from-package-expressions' and `package-derivation'.
* guix/build/gnu-build-system.scm (%parallel-job-count): New variable.
(build, check): Use it instead of $NIX_BUILD_CORES.
* guix-build.in (guix-build): Default to 0 for the #:build-cores option.
* guix-build.in (leave): New macro, formerly in `guix-build'.
(show-help): Document `--cores'.
(%options): Add `--cores'.
(guix-build): Remove `leave' macro from here. Pass the `cores' option
value to `set-build-options'.