Do not hardcode C++ library on macOS

[barracuda156]

C++ runtime library should not be hardcoded, since macOS may have either or both. Normally a compiler knows what to link to, it is not needed to specify it.

I.e. Darwin should be removed from here:

  if ['ios', 'darwin', 'android'].contains(system)
    cpp_lib = '-lc++'
  endif