Introduce gating based on MSVC warnings

This is the last compiler setup we have, that was missing a warning
counter. Initial limit we use is different than number reported by MSVC
internally, because MSVC simply sums warnings from all projects (without
removing duplicates, the way our script does).

.github/workflows/windows.yml

@@ -111,7 +111,13 @@ jobs:
           PATH: '${env:PATH};C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\MSBuild\Current\Bin\amd64'
         run: |
           cd vs
-          MSBuild -m dosbox.sln -p:Configuration=${{ matrix.type }}
+          MSBuild -m dosbox.sln -p:Configuration=${{ matrix.type }} | Tee-Object build.log
+      - name:  Summarize warnings
+        shell: pwsh
+        env:
+          MAX_WARNINGS: 401
+        run: python scripts\count-warnings.py --msvc vs\build.log
+
 
   build_windows_vs_release:
     name: Release build