use Alien::MSYS on windows where autoconf is needed

This is a patch for feature discussed in #43, to use Alien::MSYS when autoconf is detected on MSWin32.

I tested this with Acme-Alien-DontPanic.  Note that `configure` (which is what `%pconfigure` is on windows) is not the correct incantation for running `configure` on MSWin32, at least it does not work for me in Strawberry Perl.  I briefly considered changing the meaning of `%p`, but there may be others who are depending on its behavior.  I propose instead to use %c to mean run configure, and make that the new default.  This has the nice side effect that it is super easy to detect, and inject Alien::MSYS as a dep only when it is needed.