1. Open some qmake project.
2. Check which kit is configured for the project.
3. In "Tools" > "Options..." > "Build & Run" > "Kits", check which compiler will be used by this kit.
4. In "Tools" > "Options..." > "Build & Run" > "Compiler", set a different Compiler path for this compiler.
5. Build the project.
6. In Compile Output, check which compiler was used to build the project.
   For me it just used g++ and ignores the setting from step 4.

Creator should use the compiler that the user selected for compilation.