Configure: Add read_eval_file, a general purpose perl file reader/evaluator

It will return the last expression from the input file.

We also use this in read_config, which slightly changes what's
expected of Configurations/*.conf.  They do not have to assign
%targets specifically.  On the other hand, the table of configs MUST
be the last expression in each of those files.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4840)