Change _CoqProject separator settings

According to the code of coq_makefile, `\r` is not considered
white-space in _CoqProject.
(https://github.com/coq/coq/blob/a7f51315db5d70888af3b96a579eb799a2b45ca9/lib/coqProject_file.ml#L84)

coq/coq-system.el

@@ -550,7 +550,7 @@ alreadyopen is t if buffer already existed."
 				  (find-file-noselect projectfile t t))))
 	  (list projectbuffer projectbufferalreadyopen))))))
 
-(defconst coq--project-file-separator "[\r\n[:space:]]+")
+(defconst coq--project-file-separator "[\r\t\n[:space:]]+")
 
 (defconst coq--makefile-switch-arities
   '(("-R" . 2)