CI: github actions: add the output of $CC -dM -E-

Sometimes figuring what differs between platforms is useful to fix
build issues, to decide what ifdef to add for example. Let's always
call $CC -dM -E- before starting make.
diff --git a/.github/workflows/windows.yml b/.github/workflows/windows.yml
index 42bb4e8..4d14d48 100644
--- a/.github/workflows/windows.yml
+++ b/.github/workflows/windows.yml
@@ -51,6 +51,9 @@
           zlib-devel
     - name: Compile HAProxy with ${{ matrix.CC }}
       run: |
+        echo "::group::Show platform specific defines"
+        echo | ${{ matrix.CC }} -dM -xc -E -
+        echo "::endgroup::"
         make -j$(nproc) all \
           ERR=1 \
           TARGET=${{ matrix.TARGET }} \