make && make test
Created a md and was able to use md5 on it:
# mdconfig -a -t swap -s 64m
md0
# env LD_PRELOAD=/usr/obj/usr/src/lib/libmd/libmd.so md5 /dev/md0
MD5 (/dev/md0) = 462be07247e06c953b885b168d1753a1
# sh -c 'for i in /dev/md0; do echo -n "$i: "; md5 <$i; done'
/dev/md0: 462be07247e06c953b885b168d1753a1