configure.ac: Implement some portable sh programming recommendations from Autoconf. * configure.ac: Don't double quote arguments to 'test' unnecessarily. Don't double quote argument to case statement. Don't use double quotes with backticks. Don't unnecessarily quote "yes" and "no". Don't use test XXX -a YYY for AND functionality. Do use "test STR1 != STR2" rather than less clear "if test STR1 = STR2; then true; else ACTION; fi"

((nil .
      ((c-file-style . "gnu")
       (indent-tabs-mode . nil)
       (fill-column . 72)
       (eval . (when (string-match "\\.h\\'" (buffer-file-name))
                 (unless (string-match "/gnulib/" (buffer-file-name))
                   (c++-mode)
                   (c-set-style "gnu"))))))
 (change-log-mode . ((indent-tabs-mode . t)))
 (makefile-mode . ((indent-tabs-mode . t))))