packages: Define '%hurd-systems'.

* guix/packages.scm (%hurd-systems): New variable.

guix/packages.scm

@@ -94,6 +94,7 @@
             package-grafts
 
             %supported-systems
+            %hurd-systems
             %hydra-supported-systems
             supported-package?
 
@@ -190,6 +191,10 @@ representation."
   ;; expect all packages to build successfully here.
   '("x86_64-linux" "i686-linux" "armhf-linux" "mips64el-linux"))
 
+(define %hurd-systems
+  ;; The GNU/Hurd systems for which support is being developed.
+  '("i585-gnu" "i686-gnu"))
+
 (define %hydra-supported-systems
   ;; This is the list of system types for which build slaves are available.
   %supported-systems)