apr-util: Only specify --with-dbm=gdbm if gdbm support is enabled

Support for gdbm was made optional in 3260ad9e, but it was still being
used unconditionally.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>