[PATCH wayland-web] Build instructions for Ubuntu 16.04

[darxus at chaosreigns.com]

On 06/19, Bill Spitzak wrote:
> Looks good to me. 

Thanks.

> Nice to see that they updated all that x11proto stuff.

Yes!

> I like adding the error message you got to the comments.

Thanks, I like it.  There are some I'd like to see improved.

> One minor change below.

> I think these links should be sorted by time. Probably in reverse
> order: put yours first, then the Mint 17, then the Ubuntu 12.04
> last.

I agree, but decided to go with minimal change, since it's been years.  I
posted an update.  Also added --with-xserver-path to weston to get it to
pass make check.  Black_Prince helped me figure that one out.