Rework detection and handling of ndbm: we now check for a working

(library,header) combination, and configure defines variables for each
which the makefile and C file just use, rather than a bunch of HAVE_foo
switches. As a bonus, we no longer unnecessarily link every binary with
the ndbm library even though only slocal uses it. Also you can now tell
configure where to get the header/library so even if we guess wrong you
can overrule us.