The error actually comes from a dev-cpp/abseil-cpp header <absl/base/policy_checks.h>, where it checks for the GCC C++ language version looking at the __cplusplus macro.  The reason for doing so is to find out whether std::stringview is available or not.

GCC 9.2 defaults to C++14, which should satisfy the requirement, but some CMake macro sets it C++11 and it stays that way even if you have CXXCFLAGS="-std=c++14" in the environment.

This can be fixed by simply adding an extra CMake define in src_configure() with:

-DCMAKE_CXX_STANDARD=17

We can (and we should) use C++17 here instead of the minimal C++17 because we know the abseil-cpp version we have supports it and we want the std::stringview that is officially supported only in C++17