system_x11 variant - where to draw the line

[Jeremy Huddleston]

So right now the system_x11 variant exists in all the xorg library  
ports.  This covers everything that could be installed in ${x11prefix}/ 
lib with the exception of freetype, cairo, fontconfig, and png.  I did  
that since that's more or less where the boundary is between "what we  
use from x11prefix" and "what we use from prefix" is now by default  
(enforced by the lib:...:... style dependencies for xorg libs and  
port: style dependencies for the others mentioned above).  I'd like to  
expand that by adding a system_x11 variant to freetype, cairo, and  
fontconfig to allow users to use the version of these packages in  
x11prefix if they're available.  Does anyone have compelling reasons  
why not to do that which I might be missing?