build-system/gnu: Support parallel builds and tests.

* guix/build/gnu-build-system.scm (build): Add `parallel-build?'
  parameter; honor it and $NIX_BUILD_CORES.
  (check): Add `parallel-tests?' parameter; likewise.

* guix/build-system/gnu.scm (gnu-build): Add `parallel-build?' and
  `parallel-tests?' parameters.
  [builder]: Inherit them.

guix/build-system/gnu.scm

@@ -45,6 +45,7 @@
                     #:key (outputs '("out")) (configure-flags ''())
                     (make-flags ''())
                     (patches ''()) (patch-flags ''("--batch" "-p1"))
+                    (parallel-build? #t) (parallel-tests? #t)
                     (phases '%standard-phases)
                     (system (%current-system))
                     (modules '((guix build gnu-build-system)
@@ -63,7 +64,9 @@ input derivation INPUTS, using the usual procedure of the GNU Build System."
                   #:patch-flags ,patch-flags
                   #:phases ,phases
                   #:configure-flags ,configure-flags
-                  #:make-flags ,make-flags)))
+                  #:make-flags ,make-flags
+                  #:parallel-build? ,parallel-build?
+                  #:parallel-tests? ,parallel-tests?)))
 
   (build-expression->derivation store name system
                                 builder

guix/build/gnu-build-system.scm

@@ -87,13 +87,23 @@
     (format #t "configure flags: ~s~%" flags)
     (zero? (apply system* "./configure" flags))))
 
-(define* (build #:key (make-flags '()) #:allow-other-keys)
-  (zero? (apply system* "make" make-flags)))
+(define* (build #:key (make-flags '()) (parallel-build? #t)
+                #:allow-other-keys)
+  (zero? (apply system* "make"
+                `(,@(if parallel-build?
+                        `("-j" ,(getenv "NIX_BUILD_CORES"))
+                        '())
+                  ,@make-flags))))
 
 (define* (check #:key (make-flags '()) (tests? #t) (test-target "check")
+                (parallel-tests? #t)
                 #:allow-other-keys)
   (if tests?
-      (zero? (apply system* "make" test-target make-flags))
+      (zero? (apply system* "make" test-target
+                    `(,@(if parallel-tests?
+                            `("-j" ,(getenv "NIX_BUILD_CORES"))
+                            '())
+                      ,@make-flags)))
       (begin
         (format #t "test suite not run~%")
         #t)))