Allow compiling for Windows XP and higher.

This allows us to use getaddrinfo(), getnameinfo() and related functions, which
allow tinc to make connections over existing IPv6 networks. These functions are
not available on Windows 2000 however. By default, support is enabled, but when
compiling for Windows 2000 the configure switch --with-windows2000 should be
used.

Since getaddrinfo() et al. are not functions but macros on Windows, we have to
use AC_CHECK_DECLS() instead of AC_CHECK_FUNCS() in configure.in.