* guix/scripts/build.scm (%default-options): Remove 'system'.
(%options) <--system>: Keep previous occurrences of 'system in RESULT.
(options->derivations)[system]: Remove.
[systems, things-to-build]: New variables.
[compute-derivation]: New procedure.
Iterate on all of SYSTEMS to compute the derivations of THINGS-TO-BUILD.
* tests/guix-build.sh: Add test for one and multiple '-s' flags.
* doc/guix.texi (Additional Build Options): Document this behavior.
