If you are not using Tcl and Tk framework builds installed in /Library, you use the configure or make options:

  --with-tcltk-includes='-I...'
                          override search for Tcl and Tk include files
  --with-tcltk-libs='-L...'
                          override search for Tcl and Tk libs

to specify exactly where to find your versions of Tcl and Tk. See the comments in setup.py:

    def detect_tkinter_explicitly(self):
        # Build _tkinter using explicit locations for Tcl/Tk.
        #
        # This is enabled when both arguments are given to ./configure:
        #
        #     --with-tcltk-includes="-I/path/to/tclincludes \
        #                            -I/path/to/tkincludes"
        #     --with-tcltk-libs="-L/path/to/tcllibs -ltclm.n \
        #                        -L/path/to/tklibs -ltkm.n"
        #
        # These values can also be specified or overridden via make:
        #    make TCLTK_INCLUDES="..." TCLTK_LIBS="..."
        #
        # This can be useful for building and testing tkinter with multiple
        # versions of Tcl/Tk.  Note that a build of Tk depends on a particular
        # build of Tcl so you need to specify both arguments and use care when
        # overriding.

        # The _TCLTK variables are created in the Makefile sharedmods target.