Dry facts:
You've installed Qt distributions (libraries, include files, etc.) of versions 5.6 and 5.7 built to use with Visual Studio C++ 2015 compiler.
You don't have Visual Studio 2015 C++ compiler (or it is not properly set up)
You've installed MinGW 4.9.2, which is a windows port of GCC compiler.
You haven't installed Qt for MinGW
Problem:
Due to a lack of binary compatibility between C++ compilers, Qt distributions (same as any other libraries) built with one compiler cannot be linked with code compiled with another compiler. QtCreator knows this and warns you.
Possible solutions:
Install Qt for MinGW (with maintenancetool.exe in Qt folder)
Install Visual Studio 2015 (make sure you check "C++ tools" during installation)
Install both
Install other matching pairs of { compiler, Qt_distribution }
(in last two cases you can easily swap between toolchains)
Make sure you roll back any damage you've made to compiler settings in QtCreator.