This allows other packages to make additional preparations before using
the build_provider macro

Correct OPENSSL_APPLINK_SOURCE and OpenSSL::applink if they aren't set.
Correct not only OPENSSL_CRYPTO_LIBRARIES and OPENSSL_SSL_LIBRARIES,
but also OPENSSL_CRYPTO_LIBRARIES and OPENSSL_SSL_LIBRARY.

[work in progress]

This leaves it to different platforms to apply an appropriate prefix
and suffix according to their standards.

Related: https://github.com/gost-engine/engine/pull/313

If another package add_subdirectory() this one, and found OpenSSL first,
it's pointless to do the whole work again.

It was named 'prov', is now renamed to 'libprov'