Configure: handle undefined shared_target.

Configure: handle undefined shared_target.

Some very basic config targets don't defined the 'shared_target'
attribute at all.  This wasn't handled well enough in Configure.
This also cleans away an explicit reference to the ossltest engine in
Configurations/unix-Makefile.tmpl, which isn't necessary since the
build.info attributes were added.

Fixes openssl/web#197

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13031)