configure.ac (--with-diagnostics-color): New configure option, default to --with-diagnostics-color=auto.

	* configure.ac (--with-diagnostics-color): New configure
	option, default to --with-diagnostics-color=auto.
	* toplev.c (process_options): Use DIAGNOSTICS_COLOR_DEFAULT
	to determine -fdiagnostics-color= option default.
	* doc/invoke.texi (-fdiagnostics-color=): Document new
	default.
	* configure: Regenerated.
	* config.in: Regenerated.

From-SVN: r217540