fix string cast for c++20

[joequant]

No description provided.

[HDembinski]

Hi @joequant this looks reasonable, but a bit of documentation would be nice.

I see in (1) here https://en.cppreference.com/w/cpp/io/basic_stringstream/str
that the signature indeed changes with C++20. It therefore looks to me that your condition is not sufficient. This chance should affect all compilers eventually, therefore it seems _GLIBCXX_USE_CXX11_ABI is not the right thing to put into the condition.

[HDembinski]

Closing this for lack of response.