Don't export internal symbols

On Linux when creating the .so file we were exporting all symbols. We should
only be exporting public symbols. This commit fixes the issue. It is only
applicable to linux currently although the same technique may work for other
platforms (e.g. Solaris should work the same way).

This also adds symbol version information to our exported symbols.

Reviewed-by: Richard Levitte <levitte@openssl.org>