Minor fixes for c++17/c++20

[emmenlau]

[lnw]

That's certainly an improvement. It might be slightly safer + better style to use "[] (const unsigned char x) {return std::isdigit(x);}" instead. Reference: https://en.cppreference.com/w/cpp/string/byte/isdigit

[hmaarrfk]

I've also rebased https://github.com/ukoethe/vigra/pull/542 to see if things work out.

[hmaarrfk]

CIs passed in #542 thank you