Fix a possibly long-standing issue, but also possibly fallout from the db4
changes.  The "dbm" module could fail to build correctly, erroring out with:

*** WARNING: renaming "dbm" since importing it failed: build/lib.linux-i686-2.4/dbm.so: undefined symbol: dbm_firstkey

In this case, the "dbm" module has accidentally linked with "databases/gdbm",
which happens to be installed, but was never buildlinked in.  It may be
relevant that /usr/include/gdbm/ndbm.h is installed on this system.

Remove the "gdbm" test from the "dbm" module configuration, leaving the "ndbm"
support, and the fall-back "bdb" support (which will likely fall back to db4).

Bump PKGREVISION - the package would still install, but with missing
functionality.

(dsainty)