Configure - Allow CODErefs and ARRAYrefs in configuration setting arrays

This provides for more powerful lazy evaluation and buildup of the
setting contents.  For example, something like this becomes possible:

    defines => [ sub { $config{thisorthat} ? "FOO" : () } ]

Any undefined result of such functions (such as 'undef' or the empty
list) will be ignored.

Reviewed-by: Andy Polyakov <appro@openssl.org>