Easytag 1.99.11 is build without MP4/AAC Support. USE-flag "aac" is properly detected and used to add configure-option "--enable-mp4", but during build these ouput hints to the failure: checking for MP4Read in -lmp4v2... yes checking for MP4/AAC file support... yes ./configure: line 21176: mpeg4ip-config: command not found mpeg4ip-config is part of the media-video/mpeg4ip package, but that is no dependency of easytag with AAC support. After manually emerging that package and re-emerging easytag, AAC support is available.
Can't find the bug num to dup this of, but it occurs when you have had some older version of one of the AAC libs installed. You can ignore that message. If it still doesn't work after unmerging mpeg4ip please reopen this bug.