erofs-utils: fix configure.ac

./configure will fail when --with-lz4-libdir is not set, since
$with_lz4_libdir will be an empty string and generate an empty -L
into LDFLAGS. This patch fixes it.

Link: https://lore.kernel.org/r/20191122085859.GA2414688@usamimi.host.haruue.net
Signed-off-by: Haruue Icymoon <i@haruue.moe>
Fixes: d51c2d043773 ("erofs-utils: introduce lz4/lz4hc compression algorithm")
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>