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.

.cvsignore

@@ -13,7 +13,6 @@ aclocal.m4
 autom4te.cache
 b.out
 bison-*
-build-aux
 conf[0-9]*
 confdefs*
 config.cache