build-system/trivial: Implement the cross-build protocol.

* guix/build-system/trivial.scm (guile-for-build): New procedure.
  (trivial-build): Use it.
  (trivial-cross-build): New procedure.
  (trivial-build-system): Use it.

guix/build-system/trivial.scm

@@ -25,13 +25,7 @@
   #:use-module (ice-9 match)
   #:export (trivial-build-system))
 
-(define* (trivial-build store name source inputs
-                        #:key
-                        outputs guile system builder (modules '())
-                        search-paths)
-  "Run build expression BUILDER, an expression, for SYSTEM.  SOURCE is
-ignored."
-  (define guile-for-build
+(define (guile-for-build store guile system)
   (match guile
     ((? package?)
      (package-derivation store guile system))
@@ -42,14 +36,34 @@ ignored."
             (guile  (module-ref distro 'guile-final)))
        (package-derivation store guile system)))))
 
+(define* (trivial-build store name source inputs
+                        #:key
+                        outputs guile system builder (modules '())
+                        search-paths)
+  "Run build expression BUILDER, an expression, for SYSTEM.  SOURCE is
+ignored."
   (build-expression->derivation store name system builder inputs
                                 #:outputs outputs
                                 #:modules modules
-                                #:guile-for-build guile-for-build))
+                                #:guile-for-build
+                                (guile-for-build store guile system)))
+
+(define* (trivial-cross-build store name target source inputs native-inputs
+                              #:key
+                              outputs guile system builder (modules '())
+                              search-paths native-search-paths)
+  "Like `trivial-build', but in a cross-compilation context."
+  (build-expression->derivation store name system
+                                `(begin (define %target ,target) ,builder)
+                                (append native-inputs inputs)
+                                #:outputs outputs
+                                #:modules modules
+                                #:guile-for-build
+                                (guile-for-build store guile system)))
 
 (define trivial-build-system
   (build-system (name 'trivial)
                 (description
                  "Trivial build system, to run arbitrary Scheme build expressions")
                 (build trivial-build)
-                (cross-build trivial-build)))
+                (cross-build trivial-cross-build)))

tests/packages.scm

@@ -94,7 +94,7 @@
                    ("d" ,d) ("d/x" "something.drv"))
                  (pk 'x (package-transitive-inputs e))))))
 
-(test-skip (if (not %store) 5 0))
+(test-skip (if (not %store) 6 0))
 
 (test-assert "return values"
   (let-values (((drv-path drv)
@@ -203,6 +203,15 @@
     (and (derivation-path? drv-path)
          (derivation? drv))))
 
+(test-assert "package-cross-derivation, trivial-build-system"
+  (let ((p (package (inherit (dummy-package "p"))
+             (build-system trivial-build-system)
+             (arguments '(#:builder (exit 1))))))
+    (let-values (((drv-path drv)
+                  (package-cross-derivation %store p "mips64el-linux-gnu")))
+      (and (derivation-path? drv-path)
+           (derivation? drv)))))
+
 (unless (false-if-exception (getaddrinfo "www.gnu.org" "80" AI_NUMERICSERV))
   (test-skip 1))
 (test-assert "GNU Make, bootstrap"