Tclap does not compile with g++-11

[travnja3]

The new g++-11 (v11.1) fails to compile tclap 1.2 and even 1.4 in C++20 mode due to, I guess, an incorrect syntax used to prevent accidental copying of ValueArg, MultiArg. The syntax should be:

MultiArg(const MultiArg<T>& rhs); MultiArg& operator=(const MultiArg<T>& rhs);
Alternatively, these could be set to delete with C++11's syntax.

https://godbolt.org/z/94rv41aPe