Add project-roots method for project.el compatibility

  emacs-ess/ESS#786

Author: vspinu

clojure-mode.el

@@ -70,6 +70,7 @@
 (require 'align)
 (require 'subr-x)
 (require 'lisp-mnt)
+(require 'project)
 
 (declare-function lisp-fill-paragraph  "lisp-mode" (&optional justify))
 
@@ -1719,6 +1720,9 @@ Return nil if not inside a project."
     (when (> (length choices) 0)
       (car (sort choices #'file-in-directory-p)))))
 
+(cl-defmethod project-roots ((project (head clojure)))
+  (list (cdr project)))
+
 (defun clojure-project-relative-path (path)
   "Denormalize PATH by making it relative to the project root."
   (file-relative-name path (clojure-project-dir)))