* configure.in: default to '--with-gnu-as' and '--with-gnu-ld'

	* configure.in: default to '--with-gnu-as' and '--with-gnu-ld'
	if gas and ld are in the source tree and are in ${configdirs}.
	If ${use_gnu_as} or ${use_gnu_ld} are 'no', then don't set the
	the --with options (but still pass them down on the command line,
	if they were explicitly specified).

	* configure: substitute SHELL value in Makefile.in with
	${CONFIG_SHELL}