Build and link with private copy of Tcl/Tk 8.6 for the macOS 10.6+
installer. The 10.9+ installer variant already does this.  This means that
the Python 3.7 provided by the python.org macOS installers no longer need or
use any external versions of Tcl/Tk, either system-provided or
user-installed, such as ActiveTcl.
