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.

.github/workflows/vtest.yml

@@ -75,6 +75,9 @@ jobs:
       run: make -C addons/wurfl/dummy
     - 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 }} \

.github/workflows/windows.yml

@@ -51,6 +51,9 @@ jobs:
           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 }} \