build: Add 'emacs-build-system'.

* Makefile.am (MODULES): Add 'guix/build-system/emacs.scm' and
  'guix/build/emacs-build-system.scm'.
* guix/build-system/emacs.scm: New file.
* guix/build/emacs-build-system.scm: New file.
* doc/guix.texi (Build Systems): Document it.

guix/build-system/emacs.scm

@@ -0,0 +1,141 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2015 Federico Beffa <beffa@fbengineering.ch>
+;;;
+;;; 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 (guix build-system emacs)
+  #:use-module (guix store)
+  #:use-module (guix utils)
+  #:use-module (guix packages)
+  #:use-module (guix derivations)
+  #:use-module (guix search-paths)
+  #:use-module (guix build-system)
+  #:use-module (guix build-system gnu)
+  #:use-module (ice-9 match)
+  #:use-module (srfi srfi-26)
+  #:export (%emacs-build-system-modules
+            emacs-build
+            emacs-build-system))
+
+;; Commentary:
+;;
+;; Standard build procedure for Emacs packages.  This is implemented as an
+;; extension of 'gnu-build-system'.
+;;
+;; Code:
+
+(define %emacs-build-system-modules
+  ;; Build-side modules imported by default.
+  `((guix build emacs-build-system)
+    (guix build emacs-utils)
+    ,@%gnu-build-system-modules))
+
+(define (default-emacs)
+  "Return the default Emacs package."
+  ;; Lazily resolve the binding to avoid a circular dependency.
+  (let ((emacs-mod (resolve-interface '(gnu packages emacs))))
+    ;; we use 'emacs' instead of 'emacs-no-x' because the latter appears not
+    ;; to be loading some macros and causes problems to some packages.  For
+    ;; example, with the latter AUCTeX gives the error message:
+    ;; "(invalid-function dbus-ignore-errors)".
+    (module-ref emacs-mod 'emacs)))
+
+(define* (lower name
+                #:key source inputs native-inputs outputs system target
+                (emacs (default-emacs))
+                #:allow-other-keys
+                #:rest arguments)
+  "Return a bag for NAME."
+  (define private-keywords
+    '(#:target #:emacs #:inputs #:native-inputs))
+
+  (and (not target)                               ;XXX: no cross-compilation
+       (bag
+         (name name)
+         (system system)
+         (host-inputs `(,@(if source
+                              `(("source" ,source))
+                              '())
+                        ,@inputs
+
+                        ;; Keep the standard inputs of 'gnu-build-system'.
+                        ,@(standard-packages)))
+         (build-inputs `(("emacs" ,emacs)
+                         ,@native-inputs))
+         (outputs outputs)
+         (build emacs-build)
+         (arguments (strip-keyword-arguments private-keywords arguments)))))
+
+(define* (emacs-build store name inputs
+                      #:key source
+                      (tests? #t)
+                      (test-target "test")
+                      (configure-flags ''())
+                      (phases '(@ (guix build emacs-build-system)
+                                  %standard-phases))
+                      (outputs '("out"))
+                      (search-paths '())
+                      (system (%current-system))
+                      (guile #f)
+                      (imported-modules %emacs-build-system-modules)
+                      (modules '((guix build emacs-build-system)
+                                 (guix build utils)
+                                 (guix build emacs-utils))))
+  "Build SOURCE using EMACS, and with INPUTS."
+  (define builder
+    `(begin
+       (use-modules ,@modules)
+       (emacs-build #:name ,name
+                    #:source ,(match (assoc-ref inputs "source")
+                                (((? derivation? source))
+                                 (derivation->output-path source))
+                                ((source)
+                                 source)
+                                (source
+                                 source))
+                    #:configure-flags ,configure-flags
+                    #:system ,system
+                    #:test-target ,test-target
+                    #:tests? ,tests?
+                    #:phases ,phases
+                    #:outputs %outputs
+                    #:search-paths ',(map search-path-specification->sexp
+                                          search-paths)
+                    #:inputs %build-inputs)))
+
+  (define guile-for-build
+    (match guile
+      ((? package?)
+       (package-derivation store guile system #:graft? #f))
+      (#f                                         ; the default
+       (let* ((distro (resolve-interface '(gnu packages commencement)))
+              (guile  (module-ref distro 'guile-final)))
+         (package-derivation store guile system #:graft? #f)))))
+
+  (build-expression->derivation store name builder
+                                #:inputs inputs
+                                #:system system
+                                #:modules imported-modules
+                                #:outputs outputs
+                                #:guile-for-build guile-for-build))
+
+(define emacs-build-system
+  (build-system
+    (name 'emacs)
+    (description "The build system for Emacs packages")
+    (lower lower)))
+
+;;; emacs.scm ends here