It seems that the TCL-8.3.3-r2 ebuild cleans up after itself, and the TK-8.3.3 ebuild looks for the TCL-8.3.3-r2 include files in /var/tmp/portage/tcl-8.3.3-r2/work/tcl8.3.3/generic, which does not exist anymore. So TK-8.3.3 cannot build. As a workaround, I have created soft links in /var/tmp/portage/tcl-8.3.3-r2/work/tcl8.3.3 to the /usr/lib/tcl8.3/include subdirectories, and then TK-8.3.3 builds.
This bug appears to be fixed in the latest versions of the tcl and tk packages. Please make sure your portage tree is up to date and try again.