CI: Build on Windows using GCC via MinGW (#15405)

Currently it is disabled, due to broken QtWebKit on MinGW/MSYS2
(see #15405 for details)

azure-pipelines.yml

@@ -38,3 +38,42 @@ jobs:
   - script: make check
     displayName: 'Run the tests'
 
+- job: 'amd64_windows_mingw'
+  condition: eq(1, 2)
+  pool:
+    vmImage: 'vs2017-win2016'
+  variables:
+    PACMAN_PACKAGES: 'C:\tools\msys64\var\cache\pacman\pkg'
+    QT5_DIR: C:\tools\msys64\mingw64\lib\cmake
+  steps:
+  - script: choco install --no-progress msys2
+    displayName: 'Install MSYS2'
+  - task: Cache@2
+    inputs:
+      key: pacman
+      restoreKeys: pacman
+      path: $(PACMAN_PACKAGES)
+    displayName: Cache pacman packages
+  - script: pacman --version && pacman -Sy && pacman --noconfirm -S pacman-mirrors
+    workingDirectory:  C:\tools\msys64\usr\bin\
+    displayName: 'Check pacman'
+  - script:  C:\tools\msys64\usr\bin\pacman -Syu
+    displayName: 'Update MSYS2 packages'
+  - script: |
+      set PATH=C:\tools\msys64\usr\bin;%PATH%
+      pacman --noconfirm -S mingw64/mingw-w64-x86_64-toolchain
+      pacman --noconfirm -S mingw64/mingw-w64-x86_64-cmake
+      pacman --noconfirm -S mingw64/mingw-w64-x86_64-qtwebkit
+      pacman --noconfirm -S mingw64/mingw-w64-x86_64-python2
+    displayName: 'Install requirements'
+  - script: |
+      set PATH=C:\tools\msys64\usr\bin;C:\tools\msys64\mingw64\bin;%PATH%
+      g++ --version && cmake --version && qmake --version && python --version
+    displayName: 'Display tools version'
+  - script: |
+      set PATH=%PATH:C:\Program Files\Git\bin;=%
+      set PATH=%PATH:C:\Program Files\Git\usr\bin;=%
+      set PATH=C:\tools\msys64\usr\bin;C:\tools\msys64\mingw64\bin;%PATH%
+      del C:\tools\msys64\usr\bin\sh.exe
+      cmake CMakeLists.txt -G "MinGW Makefiles"
+    displayName: 'Run CMake'