Fixed Windows platform for build the library

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Related tickets | 
| License       | GNU GPLv3

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.