lang/tcl86, x11-toolkits/tk86: update to 8.6.8

Announcement: https://sourceforge.net/p/tcl/mailman/message/36166576/

While at it, rename pkg-config files to avoid conflict with Tcl/Tk 8.7 [1].

PR: 224819 [1]
Reported by: Ting-Wei Lan <lantw44@gmail.com>