security/gost-engine: add FLAVORS and openssl30 support

This change introduces FLAVORS support to make it possible
to build gost-engine branch for OpenSSL 3.0.x.

Currently supported FLAVORS=base openssl30
assuming "base" for OpenSSL 1.1.1 branch.

Note that upstream gost-engine for OpenSSL 3.1.x is not ready yet,
so currently no openssl31 flavor for the port, either.

Also note that to build gost-engine for OpenSSL 3.0.x,
one needs to install security/openssl30 beforehand
and set both of ssl=openssl30 and FLAVOR=openssl30