Add a guix.scm for easy building on Guix.

This enables the following on Guix:

To build and install a local, possibly modified checkout:
  guix package -f guix.scm

To build it, but not install it:
  guix build -f guix.scm

To enter a shell environment for development:
  guix shell

guix.scm

@@ -0,0 +1,65 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2022 Attila Lendvai <attila@lendvai.name>
+;;;
+;;; 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/>.
+
+;; To build and install:
+;;   guix package -f guix.scm
+;;
+;; To build it, but not install it:
+;;   guix build -f guix.scm
+;;
+;; To enter a shell environment for development:
+;;   guix shell
+
+(use-modules
+ (gnu packages idris)
+ (git)
+ (guix gexp)
+ (guix git)
+ (guix git-download)
+ (guix packages))
+
+(define *source-dir* (dirname (current-filename)))
+
+(define *include-worktree-changes* #false)
+
+(define (latest-git-commit-hash dir)
+  (with-repository dir repo
+    (oid->string (reference-target (repository-head repo)))))
+
+(define (current-git-branch-name dir)
+  (with-repository dir repo
+    (branch-name (repository-head repo))))
+
+(let ((template-package idris-1.3.4)
+      (branch-name (current-git-branch-name *source-dir*)))
+  (package
+    (inherit template-package)
+    (name "idris")
+    (version (git-version (package-version template-package)
+                          (string-append
+                           branch-name
+                           (if *include-worktree-changes*
+                               "-dirty"
+                               ""))
+                          (latest-git-commit-hash *source-dir*)))
+    (source (if *include-worktree-changes*
+                (local-file *source-dir*
+                            #:recursive? #t
+                            #:select? (git-predicate *source-dir*))
+                (git-checkout (url *source-dir*)
+                              (branch branch-name))))))