Add (guix build syscalls).

* guix/build/syscalls.scm, tests/syscalls.scm: New files.
* Makefile.am (MODULES): Add guix/build/syscalls.scm.
  (SCM_TESTS): Add tests/syscalls.scm.
* guix/utils.scm (%libc-errno-pointer, errno): Remove; take from (guix
  build syscalls).

tests/syscalls.scm

@@ -0,0 +1,47 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2014 Ludovic Courtès <ludo@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (test-syscalls)
+  #:use-module (guix build syscalls)
+  #:use-module (srfi srfi-64))
+
+;; Test the (guix build syscalls) module, although there's not much that can
+;; actually be tested without being root.
+
+(test-begin "syscalls")
+
+(test-equal "mount, ENOENT"
+  ENOENT
+  (catch 'system-error
+    (lambda ()
+      (mount "/dev/null" "/does-not-exist" "ext2")
+      #f)
+    (compose system-error-errno list)))
+
+(test-equal "umount, ENOENT"
+  ENOENT
+  (catch 'system-error
+    (lambda ()
+      (umount "/does-not-exist")
+      #f)
+    (compose system-error-errno list)))
+
+(test-end)
+
+
+(exit (= (test-runner-fail-count (test-runner-current)) 0))