Set of changes that permits Tk 8.5.14 to build against the released
header files of Tcl 8.6.0.  Developed and tested only on linux so far.
A step in the saga to decouple Tcl and Tk releases.
merge 8.5
merge trunk
more merging
Merge dgp-support-tcl-8.6-build branch to complete support for building
against Tcl 8.6+ headers that was begun in checkin 6445eceebbc.