[Scons-dev] How to get variables passed to Env(variables = whatever)?

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Some more info:

        for k in keys:
            print 'will try to set: %s' % k
            try:
                save[k] = self._dict[k]
            except KeyError:
                print 'and fail...: %s' % k
                # No value may have been set if they tried to pass in a
                # reserved variable name like TARGETS.
                pass

This is where it fails... why?.. I'm giving it a totally rouge name,
it can't be a reserved name or anything. In particular, I need to
allow specify $FLEX_HOME, if it's not set in current shell, or to
override it.
Unfortunately, none of *my* code runs before this point, so I cannot
affect this *decision*.

Thanks.