wordnet installed /usr/include/tk/tk.h and /usr/include/tk/tkDecls.h on my machine. However, wordnet did not install a copy of tk. But, these header files for tk claimed to be from a different version of tk than the system copy from Portage, which installs its headers in /usr/include/tk{,Decls}.h I can't tell why wordnet would even include the tk.h in its code. None of the #include <> preprocessor directives refer to <tk/tk.h> - all use tk.h. Also, configure+automake set the CFLAGS of wordnet to -I../include, not -I../include -I../include/tk. Thus, the #include directive appear to be grabbing the system tk.h (in /usr/include/tk.h) regardless. Because the existance of these header files are not used by wordnet AFAIK and they are from a slightly different version of tk than would be installed on the system, they should be removed.
Thank you for report Nathan. Fixed in wordnet-3.0-r3.