CI: capture gcc warnings/errors (#444)

Author: vitalyster

.github/gcc.json

@@ -0,0 +1,17 @@
+{
+  "problemMatcher": [
+      {
+        "owner": "gcc",
+        "pattern": [
+          {
+            "regexp": "^(.*):(\\d+):(\\d+):\\s+(?:fatal\\s+)?(warning|error):\\s+(.*)$",
+            "file": 1,
+            "line": 2,
+            "column": 3,
+            "severity": 4,
+            "message": 5
+          }
+        ]
+      }
+    ]
+}

.github/workflows/main.yml

@@ -10,7 +10,9 @@ jobs:
     - uses: actions/checkout@v2
 
     - name: Build tests
-      run: docker buildx build . --target test -t spectrum:tests
+      run: |
+        echo "::add-matcher::.github/gcc.json" 
+        docker buildx build . --target test -t spectrum:tests
     - name: Run tests
       run: docker run -tt spectrum:tests
   linux-clang: