We do have the option of leaving out all extension modules ("/p:IncludeExtensions=false"), but it would be nice to be able to build everything that doesn't require external libs.

This also be adds an option to skip only Tkinter (analogous to "/p:IncludeSSL=false"), as that's what I actually need at the moment :).  I'm not adding options for every external project individually, as most of them are not a big deal, but OpenSSL, Tcl/Tk/Tix, and (on 2.7) bsddb take a long time to compile and might be nice to leave out when they're not needed (so I also added the ability to leave out bsddb on 2.7).

Leaving out 3.4 as its project files are not amenable to this kind of change, and 3.4's days are numbered anyway.