SConstruct: Started moving most of protocol23 SConstruct into the protocol23 folder,
but it doesn't work yet.
SCons: factored-out protocol23 is working :-)

The fix was to import the top-level "Builder" function, instead of using "env.Builder",
which does variable substitution on its arguments (and has no values for $SOURCE
and $TARGET).

Also, deleted the SConsProject directory, as that info is now in the top-level script.