set project gnu++11 c++ language version

The project seems to use features from the C++11 standard. However, the build process doesn’t explicitly specify this. As a result, the compiler might use a different C++ standard depending on its version. For example, gcc13 uses gnu++17 by default.

Although I didn't see any issue when compiling and running the project with C++17, I think it would be safer to stick to a specific version of C++ in meson build script (see http://eel.is/c++draft/diff for сompatibility between standards)