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.

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

guix/scripts/build.scm

@@ -635,8 +635,7 @@ options handled by 'set-build-options-from-command-line', and listed in
 
 (define %default-options
   ;; Alist of default option values.
-  `((system . ,(%current-system))
-    (build-mode . ,(build-mode normal))
+  `((build-mode . ,(build-mode normal))
     (graft? . #t)
     (substitutes? . #t)
     (build-hook? . #t)
@@ -729,8 +728,7 @@ must be one of 'package', 'all', or 'transitive'~%")
                           rest)))
          (option '(#\s "system") #t #f
                  (lambda (opt name arg result)
-                   (alist-cons 'system arg
-                               (alist-delete 'system result eq?))))
+                   (alist-cons 'system arg result)))
          (option '("target") #t #f
                  (lambda (opt name arg result)
                    (alist-cons 'target arg
@@ -811,15 +809,22 @@ build."
        (cut package-cross-derivation <> <> triplet <>))))
 
   (define src    (assoc-ref opts 'source))
-  (define system (assoc-ref opts 'system))
   (define graft? (assoc-ref opts 'graft?))
+  (define systems
+    (match (filter-map (match-lambda
+                         (('system . system) system)
+                         (_ #f))
+                       opts)
+      (()      (list (%current-system)))
+      (systems systems)))
 
-  ;; We may get 'unbound-variable' errors while evaluating the 'inputs' fields
-  ;; of user packages.  Since 'guix build' is the primary tool for people
-  ;; testing new packages, report such errors gracefully.
-  (with-unbound-variable-handling
-   (parameterize ((%graft? graft?))
-     (append-map (match-lambda
+  (define things-to-build
+    (map (cut transform store <>)
+         (options->things-to-build opts)))
+
+  (define (compute-derivation obj system)
+    ;; Compute the derivation of OBJ for SYSTEM.
+    (match obj
       ((? package? p)
        (let ((p (or (and graft? (package-replacement p)) p)))
          (match src
@@ -858,9 +863,17 @@ package '~a' has no source~%")
                  (set-guile-for-build (default-guile))
                  (gexp->derivation "gexp" gexp
                                    #:system system))
-                            #:system system))))
-                 (map (cut transform store <>)
-                      (options->things-to-build opts))))))
+               #:system system)))))
+
+  ;; We may get 'unbound-variable' errors while evaluating the 'inputs' fields
+  ;; of user packages.  Since 'guix build' is the primary tool for people
+  ;; testing new packages, report such errors gracefully.
+  (with-unbound-variable-handling
+   (parameterize ((%graft? graft?))
+     (append-map (lambda (system)
+                   (append-map (cut compute-derivation <> system)
+                               things-to-build))
+                 systems))))
 
 (define (show-build-log store file urls)
   "Show the build log for FILE, falling back to remote logs from URLS if

tests/guix-build.sh

@@ -44,6 +44,13 @@ guix build -e '(@@ (gnu packages bootstrap) %bootstrap-guile)'
      guix build -e '(@@ (gnu packages bootstrap) %bootstrap-guile)';	\
   then exit 1; fi )
 
+# Passing one '-s' flag.
+test `guix build sed -s x86_64-linux -d | wc -l` = 1
+
+# Passing multiple '-s' flags.
+all_systems="-s x86_64-linux -s i686-linux -s armhf-linux -s aarch64-linux"
+test `guix build sed $all_systems -d | sort -u | wc -l` = 4
+
 # Check --sources option with its arguments
 module_dir="t-guix-build-$$"
 mkdir "$module_dir"