apr-util: make gdbm optional

This helps with gpl3-free builds.

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 3260ad9e)
Signed-off-by: Steve Sakoman <steve@sakoman.com>