This is an attempt to load the symbols we need from the Tkinter.tkinter
module at run time, rather than by linking at build time.

It is one way of building a manylinux wheel that can build on a basic
Manylinux docker image, and then run with the TCL / Tk library installed
on the installing user's machine.  It would also make the situation
better on OSX, where we have to build against ActiveState TCL for
compatibility with Python.org Python, but we would like to allow
run-time TCL from e.g. homebrew.

I have tested this on Debian Jessie Python 2.7 and 3.5, and on OSX 10.9
with Python 2.7.

Questions:

* Would y'all consider carrying something like this approach in
  the matplotlib source, but not enabled by default, to help building
  binary wheels?
* Do you have any better suggestions about how to do this?
* My C fu is weak; is there a way of collecting the typedefs I need from
  the TCL / Tk headers rather than copying them into the _tkagg.cpp
  source file (typdefs starting around line 52)?
* My fu for Python C extension modules is also weak; did I configure
  exceptions and handle references correctly?

Refactoring for style.  Check Python 3 filename encoding result.

Try adding defines etc for using LoadLibrary on Windows to get the TCL /
Tk routines.

Remove ifdefs that allowed not-dynamic library resolution of TCL / Tk
symbols.

Disable link to TCL / Tk libraries now we are loading symbols at
run-time.

As Christoph G found, we can't use the tkinter extension module to find
the TCL / Tk symbols on Windows, because Windows DLLs do not return the
addresses of symbols they import from GetProcAddress.

Instead, iterate through the modules loaded in the current process to
find the TCL and Tk symbols.

See:

* https://msdn.microsoft.com/en-us/library/windows/desktop/ms682621(v=vs.85).aspx
* https://msdn.microsoft.com/en-us/library/windows/desktop/ms683179(v=vs.85).aspx

Stop testing for Tk routines when one is missing.

Add parts of TCL / Tk headers needed to compile.

If this header is enough, and correct across platforms, then we should
be able to remove the complicated TCL / Tk search algorithms at build
time.

Move typedefs into Tcl / tk mini header.

Rename typedefs for clarity.

Strip older definition of Tcl_Interp.

Check all defines compatible with Tcl / Tk 8.5 and current trunk (as of
21 May 2016).

We now don't need the Tcl / Tk headers or libraries to build.

We don't need Tkinter at build time.

We could move this Tkinter version check to
`matplotlib/backends/tkagg.py` as a run-time check, but it's very
unlikely that any Python we support would be linked against Tcl / Tk <
8.4.  For example, the Python 2.5 Python.org OSX install links against
Tcl / Tk 8.5, and the default Tcl / Tk on CentOS 5.11 is 8.4.

Now we can always build TkAgg, so force building of Agg and TkAgg.

Returning 'installing' from Windows check_requirements results in the
message '[installing, installing]', because the `check` method default
return string is also 'installing'.

Check that we can correctly import tkagg on travis and appveyor.