Add dist/constant/lib to the paths in the generated lib/buildcustomize.pl

Strictly, add it to write_buildcustomize.pl, and so that the absolute path
is added to the generated file. Also add both dist/constant/lib and ext/re
to the @INC write_buildcustomize.pl to accommodate the upcoming change to
Cwd.
Add the upstream Makefile.PL for Cwd

This needs to return to the perl repository because the upcoming change to Cwd
needs to patch to include extra logic not in the auto-generated Makefile.PL
File::Spec in XS

For work purposes, last year I reimplemented parts of File::Spec in XS.
It wasn't in a CPANable form initially, and File::Spec had core/CPAN
version skew issues, so I didn't initially publish my work.  I've now
cleaned it up and rebased it on PathTools-3.40, and the patch is attached.
This is for review, and for whoever is currently maintaining File::Spec
to merge it into blead at an appropriate juncture.  (I fear it's a bit
too late to get into 5.18.)

[With a bump to a dev version on the changed .pm files to keep
t/porting/cmp_version.t sweet]
In Cwd's THX_unix_canonpath add two casts from (void*) to keep C++ sweet.

The new XS implementation of various File::Spec methods needs a couple of
casts to compile with a C++ compiler.