configure: Add variable to see if we're bootstrapping
Debugger: Style fix.
Debugger: Implement evaluation hook for CLanguageFamily.

- Add override for EvaluateExpression(), and hook it up to
  CLanguageExpressionEvaluator. Note that for the moment it assumes
  the expression will result in an integer value.
Debugger: Hook in expression evaluation window.

- Adjust expression window interface so it can report being closed
  to TeamWindow or whoever else invokes it.
- Add a menu item to Tools to request expression evaluation. This
  now attempts to get the expression evaluator for the current language,
  and subsequently opens the expression evaluation window with it.

With these changes, basic numerical expression evaluation works.
Resolution of variable values and condition operators aren't yet
handled.