[PATCH] use libnettle for MD5

Detect libnettle MD5 components and use by default instead of the
bundled Squid version or OpenSSL libcrypto.

This patch is a transition step using its MD5 capabilities by default
when present but retaining the Squid built-in MD5 for now.

There is not expected to be any run-time differences from this as the
actual implementation in nettle is the same code by same author as
bundled with Squid. Just absent the adaptations made to it over time for
Squid guidelines compliance.

NOTE: the OpenSSL MD5 definitions removed from configure.ac have not
been used in several versions due to earlier build issues.

Amos