Add mixed C++ standard consumer tests

[SRombauts]

This adds a small consumer project to check the supported C++ standard combinations.

The CI covers:

• C++11 library / C++11 consumer
• C++11 library / C++17 consumer
• C++17 library / C++17 consumer

The C++17 consumer already exercises the existing std::filesystem::path API. This gives us a place to add std::string_view coverage from #518 without requiring the library itself to be built as C++17.

[coveralls]

coverage: 99.713%. remained the same — ci-cpp11-cpp17-consumers into master