Building: make it possible to force linking with static OpenSSL libs

Very simply, support having the .a extension to denote depending on
static libraries.  Note that this is not supported on native Windows
when building shared libraries, as there is not static library then,
just an import library with the same name.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1889)