Class OverrideEnvironment

             object --+        
                      |        
SubstitutionEnvironment --+    
                          |    
                       Base --+
                              |
                             OverrideEnvironment

A proxy that overrides variables in a wrapped construction
environment by returning values from an overrides dictionary in
preference to values from the underlying subject environment.

This is a lightweight (I hope) proxy that passes through most use of
attributes to the underlying Environment.Base class, but has just
enough additional methods defined to act like a real construction
environment with overridden values.  It can wrap either a Base
construction environment, or another OverrideEnvironment, which
can in turn nest arbitrary OverrideEnvironments...

Note that we do *not* call the underlying base class
(SubsitutionEnvironment) initialization, because we get most of those
from proxying the attributes of the subject construction environment.
But because we subclass SubstitutionEnvironment, this class also
has inherited arg2nodes() and subst*() methods; those methods can't
be proxied because they need *this* object's methods to fetch the
values from the overrides dictionary.

Initialization of a basic SCons construction environment,
including setting up special construction variables like BUILDER,
PLATFORM, etc., and searching for and applying available Tools.

Note that we do *not* call the underlying base class
(SubsitutionEnvironment) initialization, because we need to
initialize things in a very specific order that doesn't work
with the much simpler base class initialization.

x.__setattr__('name', value) <==> x.name = value

Emulates the get() method of dictionaries.

Emulates the items() method of dictionaries.

Emulates the items() method of dictionaries.

Update an environment's values directly, bypassing the normal
checks that occur when users try to set items.

Replace existing construction variables in an Environment
with new construction variables and/or values.