[core] fix build --disable-ipv6 (fixes #2832)
[core] fix build --disable-ipv6 (fixes #2832)

x-ref:
  "Build error on systems without IPV6 support (regression from 1.4.46)"
  https://redmine.lighttpd.net/issues/2832

github: closes #87

lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4