Bugzilla Bug 230683

It seems like as a result of bug 202781, it is no longer possible to build
Graphviz against Cairo (for rendering support) without USE="gtk".  This would
be useful on a server where we would like to use graphviz to produce images,
but don't need GTK or the dozens of x11-libs that GTK depends on.  The cairo
USE flag was combined with the gtk flag, even though the use of cairo and gtk
should be independent.

Reproducible: Always

Steps to Reproduce:
emerge -p graphviz
Actual Results:  
x11-libs/cairo-1.6.4  
(...lots of x11-libs...)
x11-libs/gtk+-2.12.9-r2  USE="jpeg tiff -X -cups -debug -doc -vim-syntax
-xinerama" 
media-gfx/graphviz-2.18  USE="gtk*" 



Expected Results:  
x11-libs/cairo-1.6.4  
media-gfx/graphviz-2.18  USE="cairo"

readded cairo in media-gfx/graphviz-2.20.2.