Configure: reuse "cached" value of "none" for dir

Configure: reuse "cached" value of "none" for dir

When value of a directory (i.e. man1dir) is set to "none" because of:
- '-D' option set on cmdline (i.e. -Dman1dir=none) OR
- because value is reused from previous configure run

And when the prefix is different from the previous configure run(*) then
keep the value of "none". predefined/cached value does not contain the
old prefix so there is no harm in keeping it.

(*): due to another bug this also happens when prefix ends with a
     trailing slash

See https://rt.perl.org/Ticket/Display.html?id=134284