maint.mk: don't maintain a second build-aux variable.

maint.mk: don't maintain a second build-aux variable.

* maint.mk (build_aux): Removed.  The maintainer-makefile module
depends on GNUmakefile, which already maintains a cfg.mk
overridable $(_build-aux) for projects with a non-standard
build-aux directory location, although without the $(srcdir)
prefix.  Use that variable consistently instead of introducing a
second one.  Adjust all call sites.