guix-build: Add `--cores'.

* guix-build.in (leave): New macro, formerly in `guix-build'.
  (show-help): Document `--cores'.
  (%options): Add `--cores'.
  (guix-build): Remove `leave' macro from here.  Pass the `cores' option
  value to `set-build-options'.

guix-build.in

@@ -64,6 +64,12 @@ exec ${GUILE-@GUILE@} -L "@guilemoduledir@" -l "$0"    \
   ;; Alist of default option values.
   '())
 
+(define-syntax-rule (leave fmt args ...)
+  "Format FMT and ARGS to the error port and exit."
+  (begin
+    (format (current-error-port) fmt args ...)
+    (exit 1)))
+
 (define (show-version)
   (display "guix-build (@PACKAGE_NAME@) @PACKAGE_VERSION@\n"))
 
@@ -76,6 +82,8 @@ Build the given PACKAGE-OR-DERIVATION and return their output paths.\n"))
   -K, --keep-failed      keep build tree of failed builds"))
   (display (_ "
   -n, --dry-run          do not build the derivations"))
+  (display (_ "
+  -c, --cores=N          allow the use of up to N CPU cores for the build"))
   (newline)
   (display (_ "
   -h, --help             display this help and exit"))
@@ -104,6 +112,12 @@ Report bugs to: ~a.~%") "@PACKAGE_BUGREPORT@"))
         (option '(#\K "keep-failed") #f #f
                 (lambda (opt name arg result)
                   (alist-cons 'keep-failed? #t result)))
+        (option '(#\c "cores") #t #f
+                (lambda (opt name arg result)
+                  (let ((c (false-if-exception (string->number arg))))
+                    (if c
+                        (alist-cons 'cores c result)
+                        (leave (_ "~a: not a number~%") arg)))))
         (option '(#\n "dry-run") #f #F
                 (lambda (opt name arg result)
                   (alist-cons 'dry-run? #t result)))))
@@ -114,16 +128,11 @@ Report bugs to: ~a.~%") "@PACKAGE_BUGREPORT@"))
 ;;;
 
 (define (guix-build . args)
-  (let-syntax ((leave (syntax-rules ()
-                        ((_ fmt args ...)
-                         (begin
-                           (format (current-error-port) fmt args ...)
-                           (exit 1))))))
   (define (parse-options)
     ;; Return the alist of option values.
     (args-fold args %options
                (lambda (opt name arg result)
-                  (leave (_ "~A: unrecognized option") opt))
+                 (leave (_ "~A: unrecognized option~%") opt))
                (lambda (arg result)
                  (alist-cons 'argument arg result))
                %default-options))
@@ -144,7 +153,7 @@ Report bugs to: ~a.~%") "@PACKAGE_BUGREPORT@"))
                                ((p _ ...)
                                 (package-derivation %store p))
                                (_
-                                 (leave (_ "~A: unknown package") x))))
+                                (leave (_ "~A: unknown package~%") x))))
                             (_ #f))
                            opts))
          (req  (append-map (lambda (drv-path)
@@ -171,7 +180,8 @@ Report bugs to: ~a.~%") "@PACKAGE_BUGREPORT@"))
 
     ;; TODO: Add more options.
     (set-build-options %store
-                        #:keep-failed? (assoc-ref opts 'keep-failed?))
+                       #:keep-failed? (assoc-ref opts 'keep-failed?)
+                       #:build-cores (or (assoc-ref opts 'cores) 1))
 
     (or (assoc-ref opts 'dry-run?)
         (and (build-derivations %store drv)
@@ -184,4 +194,4 @@ Report bugs to: ~a.~%") "@PACKAGE_BUGREPORT@"))
                                           (derivation-path->output-path
                                            d out-name)))
                                         (derivation-outputs drv)))))
-                        drv))))))
+                       drv)))))