meson: Enable GLVND by default
Most distros these days nable GLVND by default, but when a user is compiling mesa they tend to forget about enabling it.
Now a user installing their own self-built mesa library has theri own problems, but may as well as follow the trend to make it easier.
Users that will still need to disable this:
- Windows
- glx=xlib
- glx=disabled && egl=disabled