Fix ExtUtils::Installed failure with -Duserelocatableinc

Fix ExtUtils::Installed failure with -Duserelocatableinc

This patch fixes an issue with ExtUtils::Installed when Perl is compiled with
userelocatableinc. It looks a though the issue is that `%Config` contains
local paths when built with -Duserelocatableinc, some, at least, with a
leading "./". The solution is to use `File::Spec->canonpath()` to clean up the
path before comparing two paths that are otherwise the same. A better solution
might be to use some sort of other utility function that checks that paths are
the same even if they're not spelled the same. I didn't notice such a function
in File::Spec, alas.

Also, I'm not sure what effects this change might have on VMS; it deserves
further testing there.