packages: Allow the `arguments' field to be a procedure.

* guix/packages.scm (package-derivation): When ARGS is a procedure, call
  (ARGS SYSTEM).

* distro/base.scm (gawk)[arguments]: Turn into a procedure, to disable
  tests on Cygwin.

distro/base.scm

@@ -62,9 +62,13 @@ (define-public gawk
              (nix-base32-string->bytevector
               "0sss7rhpvizi2a88h6giv0i7w5h07s2fxkw3s6n1hqvcnhrfgbb0"))))
    (build-system gnu-build-system)
-   (arguments '()
-              ;; TODO: disable tests on Cygwin
-              )
+   (arguments (case-lambda
+                ((system)
+                 (if (string=? system "i686-cygwin")
+                     '(#:tests? #f)      ; work around test failure on Cygwin
+                     '()))
+                ((system cross-system)
+                 '())))
    (inputs `(("libsigsegv" ,libsigsegv)             ; headers
              ("libsigsegv/lib" ,libsigsegv "lib"))) ; library
    (home-page "http://www.gnu.org/software/gawk/")

guix/packages.scm

@@ -171,7 +171,9 @@ (define* (package-derivation store package
               (package-source-derivation store source)
               inputs
               #:outputs outputs #:system system
-              args)))))
+              (if (procedure? args)
+                  (args system)
+                  args))))))
 
 (define* (package-cross-derivation store package)
   ;; TODO