Add configure test for the dnslookup feature

The dnslookup feature is enabled unconditionally, except for INTEGRITY.

Comment from https://codereview.qt-project.org/c/qt/qtbase/+/374549:

I think we should fix this by turning the #ifdef'ery into a CMake-time test for the necessary #includes (arpa/nameser.h, arpa/nameser_compat.h) and structures we need. The test can also determine whether the symbols we need are in the libraries the compiler links by default to or whether we need to link to (and thus load at runtime) to libresolv.so.