Error in .shlib_internal(args) : C++17 standard requested but CXX17 is not defined

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According to https://launchpad.net/~ubuntu-toolchain-r/+archive/ubuntu/test?field.series_filter=xenial, GCC 7 can be installed in Xenial with:

addons:
  apt:
    sources:
      - sourceline: 'ppa:ubuntu-toolchain-r/test'
    packages:
      - gcc-7

The compiler will be available as gcc-7.