Collapse different classes of macro databases

We have $config{openssl_algorithm_defines}, $config{openssl_other_defines}
and $config{openssl_thread_defines}.  These are treated exactly the same
in include/openssl/opensslconf.h.in, so having them separated into three
different databases isn't necessary, the reason for the separation being
long gone.  Therefore, we collapse them into one and the same,
$config{openssl_feature_defines}.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7558)