emacs config: force tabs with length equal 2 spaces in c-mode

.dir-locals.el

@@ -2,4 +2,5 @@
       ((c-file-style . "gnu")
        (indent-tabs-mode . nil)
        (fill-column . 79)))
+ (c-mode . (c-basic-offset 2))
  (makefile-mode . ((indent-tabs-mode . t))))