FindTCL: Add BSD paths for Tcl/Tk 8.6

Follow-up to commit 4a015f77 (OpenBSD: Add paths for Tcl/Tk 8.4/8.5,
2012-12-03): those paths added for OpenBSD also work on other BSDs, some
of which are already using version 8.6 of Tcl/Tk.

Modules/FindTCL.cmake

@@ -172,6 +172,8 @@ set(TCLTK_POSSIBLE_INCLUDE_PATHS
   /usr/include/tcl8.3
   /usr/include/tcl8.2
   /usr/include/tcl8.0
+  /usr/local/include/tcl8.6
+  /usr/local/include/tk8.6
   /usr/local/include/tcl8.5
   /usr/local/include/tk8.5
   /usr/local/include/tcl8.4