guix build: Accept multiple '-s' options.

* 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.

1 files changed, 3 insertions, 1 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 6b713aaf9c8..8c7522f286b 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -8030,7 +8030,9 @@ The following derivations will be built:
 @item --system=@var{system}
 @itemx -s @var{system}
 Attempt to build for @var{system}---e.g., @code{i686-linux}---instead of
-the system type of the build host.
+the system type of the build host.  The @command{guix build} command allows
+you to repeat this option several times, in which case it builds for all the
+specified systems; other commands ignore extraneous @option{-s} options.
 
 @quotation Note
 The @code{--system} flag is for @emph{native} compilation and must not