It seems that FIND_PACKAGE_HANDLE_STANDARD_ARGS is doing a case-sensitive
search for whether the find_package flags included REQUIRED.
(cherry-picked from commit f259976659)
This also removes the included SHA1 and MD5 implementations
in favour of those from OpenSSL.
Thanks to @jyrkive for helping to get the CMake build working and
ensuring the game still compiles with OpenSSL 1.1.0.
(The SHA_xxx() aliases were removed in OpenSSL 1.1.0.)