weechat build failure, tcl86

weechat picks up Tcl 8.6 if present:

-- Found Tclsh: /usr/local/bin/tclsh8.5 (found version "8.5")
-- Found TCL: /usr/local/lib/libtcl86.so.1.8
-- Found TCLTK: /usr/local/lib/libtcl86.so.1.8
-- Found TK: /usr/local/lib/libtk86.so.1.8