Add missing definitions on Windows.
Don't search in local directories for include files.

Tinc's source code doesn't rely on this anymore, and this gets rid of potential conflicts with
system headers.
Don't use vasprintf() anymore on Windows.

Windows doesn't actually support it, but MinGW provides it. However, with some versions of
MinGW it doesn't work correctly. Instead, we vsnprintf() to a local buffer and xstrdup() the
results.