gnu99 should works

Author: sdcb

.github/workflows/opencv.yml

@@ -134,7 +134,7 @@ jobs:
           elif [[ ${{ matrix.os }} == "centos.7" ]]; then
             # source /opt/rh/devtoolset-11/enable
             # echo "Doing nothing for centos.7"
-            EXTRA_FLAGS+=" -DCMAKE_C_FLAGS='-std=c99'"
+            EXTRA_FLAGS+=" -DCMAKE_C_FLAGS='-std=gnu99'"
           fi
   
           cmake ${{ env.OPENCV_CMAKE_ARGS }} $EXTRA_FLAGS ..