If USE mule is defined, there is a blank dependency check thanks to the line: mule? () I found that simply removing that line allowed XEmacs to compile properly with MULE support. Perhaps there is a better solution, however.
Also the mule flag is not described in /usr/portage/profiles/use.desc.
*** Bug 4976 has been marked as a duplicate of this bug. ***
should be fixed in -r2