xfstests: detect gdbm library correctly

xfstests: detect gdbm library correctly

Debian unstable has move the location of the gdbm libraries as part
of it's multi-arch conversion. The libraries are now only found by
the configured library search path in /etc/ld.conf.d, so change the
autoconf macros to test and find the libraries by link tests rather
than hard coding library paths when a header is found.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>