uClibc does not have res_send() implemented so 2.0.0 fails to build completely, while 1.8 works like a charm. (see https://support.zabbix.com/browse/ZBX-5197)

The res_send() was introduced as a response to https://support.zabbix.com/browse/ZBXNEXT-467 (you cannot pick DNS over tcp due to limitation in libresolv).

I suggest using the c-ares library for dns resolution. The API looks fairly similar to libresolv, so the porting should not bee a too big job.
Benefits:

• all platforms will have full DNS functionality
• you will be able to choose between TCP or UDP for DNS lookups
• since c-ares is available for both *nix and windows, you can use same code for all platforms (you can get rid of lots of the #if #else in code)

Drawbacks:

• Yet another external dependency. (curl will most likely already depend on c-ares though)

It could be added an an optional dependency but that would make the #if #else logic alot worse than today.