Introduced some bureaucracy for setting and tracking the executables

that a particular compiler system depends on.  This consists of the
'set_executables()' and 'set_executable()' methods, and a few lines in
the constructor that expect implementation classes to provide an
'executables' attribute, which we use to initialize several instance
attributes.  The default implementation is somewhat biased in favour of
a Unix/DOS "command-line" view of the world, but it shouldn't be too
hard to override this for operating systems with a more sophisticated
way of representing programs-to-execute.