Solaris

Then there is no installation problem. The library is here and it contains the required function.

So? What must I do?

Fix your software building procedure (configure and/or Makefile). It is at least missing the -lresolv flag.

It's not my software! I say it's courier-imap server.

By your software, I mean the software you are trying to build from source. You need to fix its build procedure and/or report a bug to its developer.

Ok... But if there is a problem with the standard library? There isn't a pkg it contain all of the standard library of C and C++?

No, there isn't a single package. The C standard library is always installed, being a mandatory component to run any program, i.e. not only those written in C. The operating system wouldn't run without it. On the other hand, its associated headers are optional as they are only required to build programs.

There are multiple C++ standard libraries so the situation is more complex here.

In any case, res_query isn't part of the Solaris standard library (libc) but of the resolver library (libresolv).