Perl 5.20 or later and Module::Build are required to build this module,
and IPC::Run is required to use it.  Either GnuPG v2 or GnuPG v1
(selectable at runtime) is also required.  It has not been tested with
versions of GnuPG older than 1.4.23.

PGP::Sign uses IPC::Run features that are documented as not available on
Windows (primarily higher-numbered file descriptors) and has never been
tested with Gpg4win, so will probably not work on Windows (or, for that
matter, other non-UNIX systems).
