some configury changes

[Ben Elliston]

I've just checked in a patch that looks for expect when configure is
run.  If it's not found, the configure script aborts.  This item was
in the TODO file and I've now updated that as well.

I'm going to work on a new configure test that determines the Tcl
version in use.  I think we need to (conservatively) require a base
Tcl version so that we can still be portable, but use useful new(ish)
Tcl features at the same time.  Thoughts?  How about 8.3?

Ben