message: Install libstdc++-9-dev in Ubuntu:16.04 docker as ISPC requires C++17
diff: @@ -60,7 +60,11 @@ FROM llvm_build_only AS ispc_build
  ARG LLVM_VERSION
  -RUN apt-get -y update && apt-get install -y m4 bison flex zlib1g-dev libc6-dev-i386 && \
  +# ISPC builds in C++17 mode and will fail without modern libstdc++
  +# Also install regular ISPC dependencies
  +RUN add-apt-repository ppa:ubuntu-toolchain-r/test -y && apt-get -y update && \
  + apt-get install -y libstdc++-9-dev && \
  + apt-get install -y m4 bison flex zlib1g-dev libc6-dev-i386 && \
  rm -rf /var/lib/apt/lists/*
  # Configure ISPC build
  