Replace Windows specific macro with a more generic feature detection
macro. Allows slirp smb feature to be disabled manually as well.

Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Joelle van Dyne <j@getutm.app>
Message-Id: <20210315180341.31638-5-j@getutm.app>
[Use $default_feature as the default. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>