Qt is a cross-platform UI framework, and the first Freeciv-Qt frontend was based on Qt 5.3. Check out the Qt Project for downloads, documentation, etc. There's also an official Qt wiki listing known issues for various releases.
On Windows Qt5 is not supposed to work for versions older than XP SP3. Qt 5.7 does not support the Visual Studio 2012 (or older) compilers; likewise only MinGW-w64 g++ 4.7 (or newer) is supported. The scripts to start Freeciv binaries on Windows set the
An earlier experiment to use Qt in 1999 was stopped due to license issues.