Fixed Windows platform for build the library

When the `--plat-name` option is empty the resulting tar file name:
    - Windows: <program name>-<version>.tar
    - Cygwin:  <program name>-<version>..tar

So we need to specified the platform.

Maybe it's better to use only a marker instead of the real platform.
Merge pull request #37 from alquerci/fix/win_build

Fixed Windows platform for build the library