Fixing minimal-net for Cygwin.

minimal-net did not compile under Cygwin because of a missing #ifdef __CYGWIN__.

There are still issues for many examples related to #249. The build system parameters require a clean-up throughout all platforms and examples (UIP_CONF_IPV6, WITH_UIPV6, UIP_CONF_IPV6_RPL, UIP_CONF_RPL, and make variable vs define).