The internal implementations for md5 and sha256 checksums have not been merged
from upstream sources for a long time.  Instead of us carrying copies of code
from other projects, we should just support building against multiple libraries
that provide such functionality.

This patch removes the md5 and sha2 code (originally obtained from PolarSSL)
from our repository.  The configure script will now error unless at least one
library supporting checksum generation is present, with the only library
currently supported being openssl.  It will be relatively simple for other
such libraries (e.g. nettle) to be supported if anyone wishes to add them.

Signed-off-by: Allan McRae <allan@archlinux.org>