There is a bug while emerging cups (or anything that depends on it) while having the X and cups USE-flags set). The reason is that cups depends on ghostscript that depends on gimp-print that depends on cups. This causes gimp- print to be emerged first which fails because of the cups-dependency. The workaround is to emerge ghostscript instead (which depends on gimp-print and cups). You get them in the right order since cups comes last in the chain I guess.
*** This bug has been marked as a duplicate of 8527 ***
