Add option to skip building Tcl/Tk

When building a program which doesn't use a GUI, it's sometimes useful to avoid including Tcl/Tk entirely.  This makes the program smaller for distribution when using e.g. PyInstaller, and has the side benefit of not pulling in additional GUI-related DLLs.

This PR simply adds an option to skip building Tcl/Tk.  Tested with MPL 1.3.1 and 1.4.2, Windows 7, 32-bit Python 2.7.8.