Unable to specify the compiler with CMake

Never try to set the compiler in the CMakeLists.txt file.

See the CMake FAQ about how to use a different compiler:

https://gitlab.kitware.com/cmake/community/wikis/FAQ#how-do-i-use-a-different-compiler

(Note that you are attempting method #3 and the FAQ says "(avoid)"...)

We recommend avoiding the "in the CMakeLists" technique because there are problems with it when a different compiler was used for a first configure, and then the CMakeLists file changes to try setting a different compiler... And because the intent of a CMakeLists file should be to work with multiple compilers, according to the preference of the developer running CMake.

The best method is to set the environment variables CC and CXX before calling CMake for the very first time in a build tree.

After CMake detects what compilers to use, it saves them in the CMakeCache.txt file so that it can still generate proper build systems even if those variables disappear from the environment...

If you ever need to change compilers, you need to start with a fresh build tree.

Using with FILEPATH option might work:

set(CMAKE_CXX_COMPILER:FILEPATH C:/MinGW/bin/gcc.exe)

I had similar problem as Pietro,

I am on Window 10 and using "Git Bash". I tried to execute >>cmake -G "MinGW Makefiles", but I got the same error as Pietro.

Then, I tried >>cmake -G "MSYS Makefiles", but realized that I need to set my environment correctly.

Make sure set a path to C:\MinGW\msys\1.0\bin and check if you have gcc.exe there. If gcc.exe is not there then you have to run C:/MinGW/bin/mingw-get.exe and install gcc from MSYS.

After that it works fine for me