Minify autotools further

- Drop support for plugins (If they can build the plugins, they can
  probably build CMake)
- Drop support for Windows (CMake will always work here)

Change-Id: Ic4acfb6da7836b0726378cd1c2225aff2675f79c
Reviewed-on: http://review.couchbase.org/43700
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>