Newer versions of the OpenSSL library have deprecated the MD5() API
function in favor of the EVP_* functions.  This commit replaces MD5()
with EVP_Digest().

Note: The new function generates the same hash values as the old
function.

Add a check in the configure script to make sure the OpenSSL library
does have the EVP_Digest() function that we now need.

Newer versions of the OpenSSL library have deprecated the MD5 API
functions in favor of the EVP_* functions.  This commit is essentially
the same as c9516ac, but for the unifyfs-stage util.