Streamline dependency-file generation

Streamline dependency-file generation

* configure.ac (AUTODEPEND_PARENTS): New var.
mkdir the dependency directories here, to simplify ‘make’.
Remove dependency files just before outputting Makefiles, so that
they are preserved if ‘configure’ exits early due to some other problem.
* lib/Makefile.in, lwlib/Makefile.in, oldXMenu/Makefile.in:
* src/Makefile.in: Adjust deps strategies to be similar, as follows:
(MKDEPDIR): Remove.  All uses removed.  This cuts down on the
number of processes spun off by ‘make’.
(clean mostlyclean): Remove $(DEPDIR) contents, not $(DEPDIR) itself.
(distclean): Remove $(DEPDIR) itself.
* lwlib/Makefile.in (all): Move to front, so that depdir includes
do not alter default action.