Give util/mkinstallvars.pl more fine grained control over var dependencies

Give util/mkinstallvars.pl more fine grained control over var dependencies

Essentially, we try to do what GNU does.  'prefix' is used to define the
defaults for 'exec_prefix' and 'libdir', and these are then used to define
further directory values.  util/mkinstallvars.pl is changed to reflect that
to the best of our ability.

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24687)