v3.5.0 breaks conf-perl-* package resolution on Windows #1094

conf-perl-ipc-system-simple and conf-perl-string-shellquote
(required by camlp5) have always required manual installation
on Windows. This worked fine through v3.4.8.

Since v3.5.0, two issues:

1. cpanm is no longer found in PATH:
   Can't open perl script "/cygdrive/c/Strawberry/perl/bin/cpanm":
   No such file or directory

2. Even when installing modules via curl cpanmin.us | perl,
   opam verifies conf-perl-* using its internal Cygwin perl
   (C:\.opam\.cygwin\root\bin\perl.exe) which has a separate
   @inc from the perl where modules are installed. The check
   always fails:

   command: C:\.opam\.cygwin\root\bin\perl.exe -MIPC::System::Simple -e 1
   @INC: /usr/local/lib/perl5/site_perl/5.40/...

Worked fine with v3.4.8. Current workaround: fetch cpanm via
curl and pin conf-perl-* to no-op packages.