md5sum, sha*sum: use libcrypto where available

md5sum, sha*sum: use libcrypto where available

libcrypto is generally available and has well optimized
crypto hash routines particular to various systems.
For example, testing sha1sum with openssl-1.0.0j
on an i3-2310M, gives a performance boost of about 40%:

$ time sha1sum.old --tag ~/test.iso
SHA1 (/home/padraig/test.iso) = 3c27f7ed01965fd2b89e22128fd62dc51a3bef30
real    0m4.692s
user    0m4.499s
sys     0m0.162s

$ time sha1sum.new --tag ~/test.iso
SHA1 (/home/padraig/test.iso) = 3c27f7ed01965fd2b89e22128fd62dc51a3bef30
real    0m2.685s
user    0m2.512s
sys     0m0.170s

* configure.ac: By default, enable use of libcrypto if available.
* src/local.mk: Link with libcrypto.
* NEWS: Mention the md5sum and sha*sum improvements.