configure.in: new flags --with-ld and --with-as...

configure.in: new flags --with-ld and --with-as...

	* configure.in: new flags --with-ld and --with-as, equivalent
	to setting LD and AS environment variables.  Test whether
	specified arguments are GNU commands, and report them with
	checking messages.  Use the specified AS for configure
	tests too.
	* configure: ditto
	* acconfig.h: add DEFAULT_ASSEMBLER and DEFAULT_LINKER
	* config.in: ditto
	* gcc.c (find_a_file): when looking for `as' and `ld', return
	the DEFAULT program if it exists
	* collect2.c (main): use DEFAULT_LINKER if it exists
	* gcc.c (find_a_file): the test for existence of a full
	pathname was reversed

From-SVN: r22629