Save away the environment variables we rely on

There are cases when we overwrite %ENV values, and while this is
perfectly fine on some platforms, it isn't on others, because the
Configure script isn't necessarely run in a separate process, and
thus, changing %ENV may very well change the environment of the
calling shell.  VMS is such a platform.

Furthermore, saving away values that we use also allow us to save them
in configdata.pm in an effective way, and recall those values just as
effectively when reconfiguring.  Also, this makes sure that we do use
the saved away values when reconfiguring, when the actual environment
variables might otherwise affect us.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4818)