gnu: hurd: Add hurd-core-headers package.

* gnu/packages/hurd.scm (hurd-core-headers): New variable.

gnu/packages/hurd.scm

 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2014, 2015 Manolis Fragkiskos Ragkousis <manolis837@gmail.com>
+;;; Copyright © 2014, 2015, 2016 Manolis Fragkiskos Ragkousis <manolis837@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -22,6 +22,7 @@ (define-module (gnu packages hurd)
   #:use-module (guix packages)
   #:use-module (gnu packages)
   #:use-module (guix build-system gnu)
+  #:use-module (guix build-system trivial)
   #:use-module (gnu packages flex)
   #:use-module (gnu packages bison)
   #:use-module (gnu packages perl)
@@ -185,3 +186,28 @@ (define-public hurd-minimal
      "This package provides libihash, needed to build the GNU C
 Library for GNU/Hurd.")
     (license gpl2+)))
+
+(define-public hurd-core-headers
+  (package
+    (name "hurd-core-headers")
+    (version (package-version hurd-headers))
+    (source #f)
+    (build-system trivial-build-system)
+    (arguments
+     '(#:modules ((guix build union))
+       #:builder (begin
+                   (use-modules (ice-9 match)
+                                (guix build union))
+                   (match %build-inputs
+                     (((names . directories) ...)
+                      (union-build (assoc-ref %outputs "out")
+                                   directories))))))
+    (inputs `(("gnumach-headers" ,gnumach-headers)
+              ("hurd-headers" ,hurd-headers)
+              ("hurd-minimal" ,hurd-minimal)))
+    (synopsis "Union of the Hurd headers and libraries")
+    (description
+     "This package contains the union of the Mach and Hurd headers and the
+Hurd-minimal package which are needed for both glibc and GCC.")
+    (home-page (package-home-page hurd-headers))
+    (license (package-license hurd-headers))))