我遇到这个错误，我尝试重新安装MinGW，更改路径等

I had this error and I tried reinstalling MinGW, changing Path, etc

最后，从用户和系统的Environment变量中删除GCC_EXEC_PREFIX env变量使它工作！
这也是为什么CMake说C编译器识别未知的根本原因。
在我的例子中，我认为这是因为Qt自带了MinGW。这可能是你的原因（即，mingw已经存在于其他地方）

Finally, Removing the GCC_EXEC_PREFIX env variable made from the Environment variable for both User and System made it work! And this was also the root cause of the reason why CMake said C compiler identification is unknown. In my case I think it was because Qt ships with its own MinGW. That could be the reason for you too (ie, mingw already exists elsewhere)