winbuild: Clean up formatting and variable naming

- Changed space usage to line up with the whole file
- Renamed CFLAGS_SSPI/IPV6 to SSPI/IPV6_CFLAGS to be
  consistent with the other CFLAGS_x variables
- Make use of existing CFLAGS_IPV6 (previously IPV6_CFLAGS)
  instead of appending directly to CFLAGS