build: emacs-utils: Add 'emacs-byte-compile-directory'.

* guix/build/emacs-utils.scm (emacs-byte-compile-directory): New procedure.

guix/build/emacs-utils.scm

@@ -22,6 +22,7 @@
             emacs-batch-eval
             emacs-batch-edit-file
             emacs-generate-autoloads
+            emacs-byte-compile-directory
             emacs-substitute-sexps
             emacs-substitute-variables))
 
@@ -57,6 +58,16 @@
                   (update-directory-autoloads ,directory))))
     (emacs-batch-eval expr)))
 
+(define* (emacs-byte-compile-directory dir #:optional (dependency-dirs '()))
+  "Byte compile all files in DIR and its sub-directories.  Before compiling
+the files, add DIR and all directories in DEPENDENCY-DIRS to 'load-path'."
+  (let ((expr `(progn
+                (add-to-list 'load-path ,dir)
+                (when ',dependency-dirs
+                  (setq load-path (append ',dependency-dirs load-path)))
+                (byte-recompile-directory (file-name-as-directory ,dir) 0))))
+    (emacs-batch-eval expr)))
+
 (define-syntax emacs-substitute-sexps
   (syntax-rules ()
     "Substitute the S-expression immediately following the first occurrence of