Finish the transition config -> build-aux.

* configure.ac, Makefile.am: Use build-aux.
* config/prev-version, config/announce-gen, config/Makefile.am:
Move to...
* build-aux/prev-version, build-aux/announce-gen,
* build-aux/Makefile.am: here.

build-aux/.cvsignore

@@ -0,0 +1,13 @@
+*.t
+Makefile
+Makefile.in
+config.guess
+config.rpath
+config.sub
+depcomp
+install-sh
+mdate-sh
+missing
+mkinstalldirs
+texinfo.tex
+ylwrap