Dynamically infer essr version when essr-version is nil

  When ESS is installed from MELPA essr-version is not set
  #1163 (comment)

lisp/ess-r-mode.el

@@ -1554,7 +1554,12 @@ download step is omitted. This function returns t if the ESSR
 load is successful, and nil otherwise."
   (let ((loader (ess-file-content (expand-file-name "ESSR/LOADREMOTE" ess-etc-directory))))
     (or (with-temp-message "Fetching and loading ESSR into the remote ..."
-          (ess-boolean-command (format loader essr-version)))
+          (let ((essr (or essr-version
+                          ;; FIXME: Hack: on MELPA essr-version is not set
+                          (lm-with-file (expand-file-name "ess.el" ess-lisp-directory)
+                            (lm-header "ESSR-Version"))
+                          (error "`essr-version' could not be automatically inferred from ess.el file"))))
+            (ess-boolean-command (format loader essr-version))))
         (let ((errmsg (with-current-buffer " *ess-command-output*" (buffer-string))))
           (message (format "Couldn't load or download ESSR.rds on the remote.\n Error: %s\n Injecting local copy of ESSR." errmsg))
           nil))))

lisp/ess.el

@@ -51,19 +51,17 @@
 
 (require 'ess-utils)
 (require 'cl-generic)
-
+(require 'lisp-mnt)
 (defvar reporter-prompt-for-summary-p)
 
 
 ;; Versions
 
 (defconst ess-version (eval-when-compile
-                        (require 'lisp-mnt)
                         (lm-version (or load-file-name buffer-file-name)))
   "Version of ESS currently loaded.")
 
 (defconst essr-version (eval-when-compile
-                         (require 'lisp-mnt)
                          (lm-with-file (or load-file-name buffer-file-name)
                            (lm-header "ESSR-Version")))
   "Version of ESSR package.")