MSWin32 - Do not install Alien::MSYS if $use_system_ffi

[kmx]

Hi,

I would like to propose to not install Alien::MSYS if the libffi is already installed on the system.

The patch to Makefile.PL should look like this:

- if ( $^O eq 'MSWin32' ) {
+ if ( $^O eq 'MSWin32' && !$use_system_ffi ) {
        $WriteMakefileArgs{PREREQ_PM}{'Alien::MSYS'} = '0';
  }

But I have no idea how to handle this with Dist::Zilla

[plicease]

I agree with this and I think I can make the change and test it.