Upgrade C libraries in benchmark job

.circleci/config.yml

@@ -122,6 +122,14 @@ jobs:
       - checkout
       - attach_workspace:
           at: .
+      - run:
+          name: dependencies
+          command: |
+            sudo apt-get update
+            sudo apt-get install -t testing g++-8
+            sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-6 60 --slave /usr/bin/g++ g++ /usr/bin/g++-6
+            sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 80 --slave /usr/bin/g++ g++ /usr/bin/g++-8
+            sudo update-alternatives --set gcc /usr/bin/gcc-8
       - run:
           name: benchmark
           command: |