The 8.5 slotted dev-lang/tcl ebuild only installs the libs, but the headers are missing: $ qlist dev-lang/tcl:8.5 /usr/lib64/libtcl8.5.so /usr/lib64/libtclstub8.5.a /usr/lib64/tcl8.5/tclConfig.sh The headers are supposed to be in /usr/include/tcl8.5. I reckon this slotted version is provided for backwards compatibility. I'm glad to have that, but I need to compile stuff for 8.5, too, so I need the headers. Same thing goes for dev-lang/tk:8.5 by the way.
you should mask the 8.6 and emerge without specifying the slot