Run also in C11 mode for CI

.github/workflows/ccpp.yml

@@ -13,8 +13,11 @@ jobs:
     - uses: actions/checkout@v1
     - name: make
       run: make
-    - name: make check
+    - name: make check C99
       run: make check
+    - name: make check C11
+      working-directory: ./tests
+      run: make clean check CC="cc -std=c11"
 
   build_windows_mingw:
     strategy: