ox: Tolerate included file names without double quotes

* lisp/ox.el (org-export-expand-include-keyword): Tolerate included
  file names without double quotes.

lisp/ox.el

@@ -3256,19 +3256,26 @@ working directory.  It is used to properly resolve relative
 paths."
   (let ((case-fold-search t))
     (goto-char (point-min))
-    (while (re-search-forward "^[ \t]*#\\+INCLUDE: +\\(.*\\)[ \t]*$" nil t)
-      (when (eq (org-element-type (save-match-data (org-element-at-point)))
-		'keyword)
+    (while (re-search-forward "^[ \t]*#\\+INCLUDE:" nil t)
+      (let ((element (save-match-data (org-element-at-point))))
+	(when (eq (org-element-type element) 'keyword)
 	  (beginning-of-line)
 	  ;; Extract arguments from keyword's value.
-	(let* ((value (match-string 1))
+	  (let* ((value (org-element-property :value element))
 		 (ind (org-get-indentation))
-	       (file (and (string-match "^\"\\(\\S-+\\)\"" value)
-			  (prog1 (expand-file-name (match-string 1 value) dir)
-			    (setq value (replace-match "" nil nil value)))))
+		 (file (let ((f (if (eq (aref value 0) ?\") (read value)
+				  (and (string-match "^\\S-+" value)
+				       (match-string 0 value)))))
+			 (setq value
+			       (progn
+				 (string-match (format "^\"?%s\"?[ \t]*" f)
+					       value)
+				 (replace-match "" nil nil value)))
+			 (expand-file-name f dir)))
 		 (lines
 		  (and (string-match
-		      ":lines +\"\\(\\(?:[0-9]+\\)?-\\(?:[0-9]+\\)?\\)\"" value)
+			":lines +\"\\(\\(?:[0-9]+\\)?-\\(?:[0-9]+\\)?\\)\""
+			value)
 		       (prog1 (match-string 1 value)
 			 (setq value (replace-match "" nil nil value)))))
 		 (env (cond ((string-match "\\<example\\>" value) 'example)
@@ -3288,8 +3295,9 @@ paths."
 	    ;; Remove keyword.
 	    (delete-region (point) (progn (forward-line) (point)))
 	    (cond
-	   ((not file) (error "Invalid syntax in INCLUDE keyword"))
-	   ((not (file-readable-p file)) (error "Cannot include file %s" file))
+	     ((not file) nil)
+	     ((not (file-readable-p file))
+	      (error "Cannot include file %s" file))
 	     ;; Check if files has already been parsed.  Look after
 	     ;; inclusion lines too, as different parts of the same file
 	     ;; can be included too.
@@ -3322,7 +3330,7 @@ paths."
 		   (org-export-expand-include-keyword
 		    (cons (list file lines) included)
 		    (file-name-directory file))
-		 (buffer-string))))))))))))
+		   (buffer-string)))))))))))))
 
 (defun org-export--prepare-file-contents (file &optional lines ind minlevel)
   "Prepare the contents of FILE for inclusion and return them as a string.