resolve demo-debian-bullseye cxx11 abi string issue

Somewhere in the toolchain there's a flag missing resulting in:

```
client.cc:(.text+0x27fe): undefined reference to `google::cloud::v2_34::Project::Project(std::__cxx11::basic_string, std::allocator >)'

```

Start by reproducing the linker error in the demo-debian-bullseye environment and inspect the toolchain and build configuration for the missing ABI setting. Done means the demo links successfully without the undefined reference to the C++11 ABI string constructor.

Written by the indexing model from the issue text.