remove windows-only code

Step 1 on road towards Linux.

Remove windows-specific code, stub it out where needed and make sure we
can still compile it the way it is.

Take care of Travis' build matrix to include new build configuration.

Install new packages.

src/network/network.cpp

@@ -18,7 +18,9 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *****************************************************************************/
 
-#include <windows.h>
+extern "C" {
+#include "../platform/platform.h"
+}
 
 #include "network.h"