MSVC + mingw-w64 gfortran gives "gfortran.exe: error: /machine:x64: No such file or directory"

I am trying to use MSVC as the C and C++ compiler and (any) gfortran as the Fortran compiler.

The furthest that I got is by installing the m2w64-toolchain Conda package, and:

    call "c:\Program Files (x86)\Microsoft Visual Studio\2017\Community\VC\Auxiliary\Build\vcvarsall.bat" amd64
    set FC=gfortran
    set CXX=cl
    set CC=cl
    cmake -G "Ninja" .

Which gives:

**********************************************************************
** Visual Studio 2017 Developer Command Prompt v15.5.7
** Copyright (c) 2017 Microsoft Corporation
**********************************************************************
[vcvarsall.bat] Environment initialized for: 'x64'
-- The C compiler identification is MSVC 19.12.25835.0
-- The CXX compiler identification is MSVC 19.12.25835.0
-- The Fortran compiler identification is GNU 5.3.0
-- Check for working C compiler: C:/Program Files (x86)/Microsoft Visual Studio/2017/Community/VC/Tools/MSVC/14.12.25827/bin/Hostx64/x64/cl.exe
-- Check for working C compiler: C:/Program Files (x86)/Microsoft Visual Studio/2017/Community/VC/Tools/MSVC/14.12.25827/bin/Hostx64/x64/cl.exe -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2017/Community/VC/Tools/MSVC/14.12.25827/bin/Hostx64/x64/cl.exe
-- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2017/Community/VC/Tools/MSVC/14.12.25827/bin/Hostx64/x64/cl.exe -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Check for working Fortran compiler: C:/Users/XXX/miniconda3/envs/m2w64/Library/mingw-w64/bin/gfortran.exe
-- Check for working Fortran compiler: C:/Users/XXX/miniconda3/envs/m2w64/Library/mingw-w64/bin/gfortran.exe  -- broken
CMake Error at C:/Users/XXX/miniconda3/envs/m2w64/Library/share/cmake-3.9/Modules/CMakeTestFortranCompiler.cmake:44 (message):
  The Fortran compiler
  "C:/Users/XXX/miniconda3/envs/m2w64/Library/mingw-w64/bin/gfortran.exe"
  is not able to compile a simple test program.

  It fails with the following output:

   Change Dir: C:/Users/XXX/GitLab-Runner/builds/1236142e/.../CMakeFiles/CMakeTmp

  

  Run Build
  Command:"C:/Users/XXX/miniconda3/envs/m2w64/Library/bin/ninja.exe"
  "cmTC_40444"

  [1/4] Building Fortran preprocessed
  CMakeFiles/cmTC_40444.dir/testFortranCompiler.f-pp.f

  [2/4] Generating Fortran dyndep file CMakeFiles/cmTC_40444.dir/Fortran.dd

  [3/4] Building Fortran object
  CMakeFiles/cmTC_40444.dir/testFortranCompiler.f.obj

  [4/4] Linking Fortran executable cmTC_40444.exe

  FAILED: cmTC_40444.exe 

  cmd.exe /C "cd .  &&
  C:\Users\XXX\miniconda3\envs\m2w64\Library\mingw-w64\bin\gfortran.exe
  /machine:x64 CMakeFiles/cmTC_40444.dir/testFortranCompiler.f.obj -o
  cmTC_40444.exe -Wl,--out-implib,libcmTC_40444.dll.a
  -Wl,--major-image-version,0,--minor-image-version,0 && cd ."

  gfortran.exe: error: /machine:x64: No such file or directory

  ninja: build stopped: subcommand failed.

  

  

  CMake will not be able to correctly generate this project.
Call Stack (most recent call first):
  CMakeLists.txt:5 (project)


-- Configuring incomplete, errors occurred!

The same happens with the NMake Makefiles generator. Essentially, cmake passes /machine:x64 to gfotran which it doesn't understand. Why is cmake doing it? What am I doing wrong?