Bug#966270: seqan3 FTBFS with gcc 10

[Michael Crusoe]

Reply from upstream

https://gitter.im/seqan/Lobby?at=5f1e97ece9066820051dc93e

> You can fix that reported error by specifying `cmake
-DCMAKE_CXX_FLAGS="-std=c++17 -fconcepts" <...>`