Apparently that wants a PPA.

Author: BillyONeal

azure-pipelines.yml

@@ -171,8 +171,9 @@ jobs:
       vmImage: 'Ubuntu 16.04'
     steps:
     - script: |
+        sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
         sudo apt-get -y update
-        sudo apt-get install g++-6 -y
+        sudo apt-get install g++-7 -y
         ./vcpkg/bootstrap-vcpkg.sh
         ./vcpkg/vcpkg install zlib openssl boost-system boost-date-time boost-regex websocketpp boost-thread boost-filesystem boost-random boost-chrono brotli
       displayName: Vcpkg install dependencies