CMake in Visual Studio: MinGW configuration targeting both x64 and i686

CMakeSettings.json

{
  "configurations": [
    {
      "environments": [
        {
          "MINGW64_ROOT": "C:\\msys64\\mingw64",
          "BIN_ROOT": "${env.MINGW64_ROOT}\\bin",
          "FLAVOR": "x86_64-w64-mingw32",
          "TOOLSET_VERSION": "7.3.0",
          "INCLUDE": "${env.MINGW64_ROOT}\\include\\c++\\${env.TOOLSET_VERSION};${env.MINGW64_ROOT}\\include\\c++\\${env.TOOLSET_VERSION}\\tr1;${env.MINGW64_ROOT}\\include\\c++\\${env.TOOLSET_VERSION}\\${env.FLAVOR}",

          "MINGW_PREFIX": "C:/msys64/mingw64",
          "MINGW_CHOST": "x86_64-w64-mingw32",
          "MINGW_PACKAGE_PREFIX": "mingw-w64-x86_64",
          "MSYSTEM": "MINGW64",
          "MSYSTEM_CARCH": "x64_64",
          "MSYSTEM_PREFIX": "C:/msys64/mingw64",
          "MSYSTEM_CHOST": "x86_64-w64-mingw32",
          "SHELL": "${env.MINGW_PREFIX}/../usr/bin/bash",
          "TEMP": "${env.MINGW_PREFIX}/../tmp",
          "TMP": "${env.TEMP}",
          "PATH": "${env.MINGW_PREFIX}/bin;${env.MINGW_PREFIX}/../usr/local/bin;${env.MINGW_PREFIX}/../usr/bin;${env.MINGW_PREFIX}/../bin;${env.PATH}",

          "environment": "mingw_64"
        }
      ],
      "name": "Mingw64-Debug",
      "generator": "Ninja",
      "configurationType": "Debug",
      "inheritEnvironments": [
        "mingw_64", "msvc_x64_x64"
      ],
      "buildRoot": "${env.USERPROFILE}\\CMakeBuilds\\${workspaceHash}\\build\\${name}",
      "installRoot": "${env.USERPROFILE}\\CMakeBuilds\\${workspaceHash}\\install\\${name}",
      "cmakeCommandArgs": "",
      "buildCommandArgs": "-v",
      "ctestCommandArgs": "",
      "intelliSenseMode": "linux-gcc-x64",
      "variables": [
        {
          "name": "CMAKE_C_COMPILER",
          "value": "${env.BIN_ROOT}\\gcc.exe"
        },
        {
          "name": "CMAKE_CXX_COMPILER",
          "value": "${env.BIN_ROOT}\\g++.exe"
        }
      ]
    },

    {
      "environments": [
        {
          "MINGW32_ROOT": "C:\\msys64\\mingw32",
          "BIN_ROOT": "${env.MINGW32_ROOT}\\bin",
          "FLAVOR": "i686-w64-mingw32",
          "TOOLSET_VERSION": "7.3.0",
          "INCLUDE": "${env.MINGW32_ROOT}\\include\\c++\\${env.TOOLSET_VERSION};${env.MINGW32_ROOT}\\include\\c++\\${env.TOOLSET_VERSION}\\tr1;${env.MINGW32_ROOT}\\include\\c++\\${env.TOOLSET_VERSION}\\${env.FLAVOR}",

          "MINGW_PREFIX": "C:/msys64/mingw32",
          "MINGW_CHOST": "i686-w64-mingw32",
          "MINGW_PACKAGE_PREFIX": "mingw-w64-i686",
          "MSYSTEM": "MINGW32",
          "MSYSTEM_CARCH": "i686",
          "MSYSTEM_PREFIX": "C:/msys64/mingw32",
          "MSYSTEM_CHOST": " i686-w64-mingw32",
          "SHELL": "${env.MINGW_PREFIX}/../usr/bin/bash",
          "TEMP": "${env.MINGW_PREFIX}/../tmp",
          "TMP": "${env.TEMP}",
          "PATH": "${env.MINGW_PREFIX}/bin;${env.MINGW_PREFIX}/../usr/local/bin;${env.MINGW_PREFIX}/../usr/bin;${env.MINGW_PREFIX}/../bin;${env.PATH}",

          "environment": "mingw32_i686"
        }
      ],
      "name": "MinGW32-i686-Debug",
      "generator": "Ninja",
      "configurationType": "Debug",
      "inheritEnvironments": [
        "mingw32_i686", "msvc_x86_x64"
      ],
      "buildRoot": "${env.USERPROFILE}\\CMakeBuilds\\${workspaceHash}\\build\\${name}",
      "installRoot": "${env.USERPROFILE}\\CMakeBuilds\\${workspaceHash}\\install\\${name}",
      "cmakeCommandArgs": "",
      "buildCommandArgs": "-v",
      "ctestCommandArgs": "",
      "intelliSenseMode": "linux-gcc-x86",
      "variables": [
        {
          "name": "CMAKE_C_COMPILER",
          "value": "${env.BIN_ROOT}\\gcc.exe"
        },
        {
          "name": "CMAKE_CXX_COMPILER",
          "value": "${env.BIN_ROOT}\\g++.exe"
        }
      ]
    }

  ]
}

launch.vs.json

{
  "version": "0.2.1",
  "defaults": {},
  "configurations": [
    {
      "project": "CMakeLists.txt",
      "projectTarget": "CMakeProject1.exe (CMakeProject1\\CMakeProject1.exe)",
      "name": "CMakeProject1.exe (CMakeProject1\\CMakeProject1.exe)",
      "type": "cppdbg",
      "miDebuggerPath": "${env.MINGW_PREFIX}\\bin\\gdb.exe",
      "inheritEnvironments": [
        "mingw_64"
      ],
      "program": "${debugInfo.target}",
      "MIMode": "gdb",
      "externalConsole": true
    },
    {
      "project": "CMakeLists.txt",
      "projectTarget": "CMakeProject1.exe (CMakeProject1\\CMakeProject1.exe)",
      "name": "i686 - CMakeProject1.exe (CMakeProject1\\CMakeProject1.exe)",
      "type": "cppdbg",
      "miDebuggerPath": "${env.MINGW_PREFIX}\\bin\\gdb.exe",
      "inheritEnvironments": [
        "mingw32_i686"
      ],
      "program": "${debugInfo.target}",
      "MIMode": "gdb",
      "externalConsole": true
    }
  ]
}

To setup the MinGW environment, I followed these steps:

1. Install MinGW msys2-x86_64-20180531.exe from http://www.msys2.org/
2. Follow instructions on that page
3. Install devel packages:

pacman -S make gettext base-devel
pacman -S mingw-w64-x86_64-gcc
pacman -S mingw-w64-i686-gcc
pacman -S mingw-w64-x86_64-ninja mingw-w64-i686-ninja
pacman -S mingw-w64-x86_64-gdb mingw-w64-i686-gdb