When emerging tix 8.2.0 it doesn't install the header files, so you can't build any applications which want to link against tix. Investigating the image directory during the build process gives a usr/include file, but this is a _file_ and not a directory with files in it. The file seems to contain tix.h, which should be copied to an include dir, but is now skipped.
Ugly. I love Makefiles that assume directories will just be there. Fixed in CVS. Thanks for the bug report.