meson: use required : feature option correclty

The code here is a complicated way to write exactly the same thing. If
`get_option('x11')` returns `disabled`, ie `-Dx11=disabled` then
`dependency()` short circuits and returns a not found object (just like
`dependency('', required : false)`, but with the correct name for error
messages).
14 jobs for submit/fix-get-option-x11 in 3 minutes and 52 seconds (queued for 1 second)