Build: Avoid SHA256_Init on FreeBSD and MINIX 3.

Build: Avoid SHA256_Init on FreeBSD and MINIX 3.

On FreeBSD 10 and older, SHA256_Init from libmd conflicts
with libcrypto from OpenSSL. The OpenSSL version has
different sizeof(SHA256_CTX) and it can cause weird
problems if wrong SHA256_Init gets used.

Looking at the source, MINIX 3 seems to have a similar issue but
I'm not sure. To be safe, I disabled SHA256_Init on MINIX 3 too.

NetBSD has SHA256_Init in libc and they had a similar problem,
but they already fixed it in 2009.

Thanks to Jim Wilcoxson for the bug report that helped
in finding the problem.