gexp: Resolve the default system at '>>=' time.

Partly fixes <http://bugs.gnu.org/18002>.
Reported by David Thompson <dthompson2@worcester.edu>.

* guix/gexp.scm (gexp->derivation): Change #:system to default #f.
  Use (%current-system) from within the 'mlet*'.
* tests/gexp.scm ("gexp->derivation, default system"): New test.

guix/gexp.scm

@@ -94,7 +94,7 @@ input list as a monadic value."
 
 (define* (gexp->derivation name exp
                            #:key
-                           (system (%current-system))
+                           system
                            hash hash-algo recursive?
                            (env-vars '())
                            (modules '())
@@ -114,6 +114,7 @@ The other arguments are as for 'derivation'."
   (define outputs (gexp-outputs exp))
 
   (mlet* %store-monad ((inputs   (lower-inputs (gexp-inputs exp)))
+                       (system -> (or system (%current-system)))
                        (sexp     (gexp->sexp exp))
                        (builder  (text-file (string-append name "-builder")
                                             (object->string sexp)))

tests/gexp.scm

@@ -211,6 +211,17 @@
     (return (string=? (readlink (string-append out "/foo"))
                       guile))))
 
+(test-assertm "gexp->derivation, default system"
+  ;; The default system should be the one at '>>=' time, not the one at
+  ;; invocation time.  See <http://bugs.gnu.org/18002>.
+  (let ((system (%current-system))
+        (mdrv   (parameterize ((%current-system "foobar64-linux"))
+                  (gexp->derivation "foo"
+                                    (gexp
+                                     (mkdir (ungexp output)))))))
+    (mlet %store-monad ((drv mdrv))
+      (return (string=? system (derivation-system drv))))))
+
 (define shebang
   (string-append (derivation->output-path guile-for-build)
                  "/bin/guile --no-auto-compile"))