A simpler way to make the "arch" build directory

A simpler way to make the "arch" build directory

This implements a simpler way to make the "arch" build directory --
namely, now it is done as an order-only dependency in the Makefile,
rather than being created when config.status is run.  This simpler
because it means that the build directories can be changed without
re-running autoconf.

ChangeLog
2017-11-27  Tom Tromey  <tom@tromey.com>

	* configure.ac (CONFIG_SRC_SUBDIR): Don't subst.
	* configure: Rebuild.
	* Makefile.in (CONFIG_SRC_SUBDIR): Redefine.
	(CONFIG_DEP_SUBDIR): New variable.
	(%.o): Add order-only dependency.
	($(CONFIG_DEP_SUBDIR)): New target.