checking for berkeley DB 4.x within configure.in

[Adam Monsen]

Figured this one out.
(see 
http://groups.google.com/groups?selm=9b12ed8d.0405212352.37ed6969%40posting.google.com)

1. copied ax_path_bdb.m4 and ax_compare_version.m4 from
http://ac-archive.sourceforge.net/ to m4/ directory in project source
root.
2. added 'AX_PATH_BDB([4])' to top-level configure.ac
3. added 'ACLOCAL_AMFLAGS = -I m4' to top-level Makefile.am

Done! aclocal.m4 is properly rebuilt with subsequent invocations of
'autoreconf', and the 'configure' script checks for Berkeley DB >= 4
as expected.