Remove trailing '/' from prefix

Remove trailing '/' from prefix

Change how/when the trailing '/' from a prefix is removed.

The original code had two problems:

1) it only stripped the trailing slash when value was predefined
   (either from -Dprefix=/foo/bar/ or from a previous configure run)
2) when it did strip the value then it also caused 'oldprefix' to
   be set. This in turns causes other predefined (/cached) paths to
   be ignored.

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