Python compiled from Python.org source builds the tkinter module as a
built-in module, not an external module, as is the case for the packaged
builds of Debian etc:
>>> Tkinter.tkinter
<module '_tkinter' (built-in)>
This breaks the current algorithm for searching for tkinter symbols,
which loaded the external module .so file to get the symbols.
Try searching in the main program namespace for the tkinter symbols,
before looking for the extermal module .so file.
Thanks to github user ettaka for reporting : see
https://github.com/matplotlib/matplotlib/issues/7428
Discover Tcl / Tk functions at run-time by looking in libs loaded by
Tkinter Python module.
Use header excerpts from Tcl / Tk so we don't need an installed Tcl / Tk
to build.
