Tix was finally added to the pcbuild solution for 3.5 a couple months
ago, until that point it was never built on the buildbots.  If my
understanding of history is correct, build_tkinter.py has never been
used regularly, but was an initial push towards building Tcl/Tk/Tix
the same way OpenSSL is built in <3.4.  See #15968 and #21141 if you
want more information on the situation in 3.5.  The buildbot scripts
for 2.7 and 3.4 do build usable, debug versions of Tcl and Tk, though.
 The lack of Tix is hard to notice since there are absolutely no tests
for it yet.

I'm wary of backporting the Tcl/Tk/Tix building changes, especially to
2.7, but it should be possible to tack Tix onto the current buildbot
scripts.  I'll open an issue for that.