+system_x11 to bite the dust

[Jeremy Huddleston]

So, the MacPorts-provided X11 libs should be regression free over what  
is installed in any x11prefix.  The hardware-rendering libGL was the  
last nail, and that's been fairly stable for the past 1-2 months.

Can we successfully nuke the backwards-compatible system_x11 variant  
now?  If any cares to answer "no," then you also need to explain why,  
so I can make sure to address your concerns before pulling the plug.

"no, I like not using MP's X11 libs" is not a valid reason.

"wait, wait... the XFree86 port still uses x11prefix" is also not a  
valid reason.  That port can be updated to be the *only* port using  
x11prefix (as a local variable).  It will install the server that pure  
darwin users will need in this alternate prefix, and any other X11  
application installed by MacPorts will use the new hotness X11 libs.

After this is done, I'm going to go through and start removing all the  
x11prefix references and workarounds in various ports.  If you are  
maintainer on a !openmaintainer port that has reference to $ 
{x11prefix}, speak now, or I consider your silence approval.

Once that is done, we can update base to no longer need x11prefix...  
and our lives should be much simpler...

--Jeremy