Useful future statement support for the interactive interpreter

(Also remove warning about module-level global decl, because we can't
distinguish from code passed to exec.)

Define PyCompilerFlags type contains a single element,
cf_nested_scopes, that is true if a nested scopes future statement has
been entered at the interactive prompt.

New API functions:
    PyNode_CompileFlags()
    PyRun_InteractiveOneFlags()
    -- same as their non Flags counterparts except that the take an
       optional PyCompilerFlags pointer

compile.c: In jcompile() use PyCompilerFlags argument.  If
    cf_nested_scopes is true, compile code with nested scopes.  If it
    is false, but the code has a valid future nested scopes statement,
    set it to true.

pythonrun.c: Create a new PyCompilerFlags object in
    PyRun_InteractiveLoop() and thread it through to
    PyRun_InteractiveOneFlags().
diff --git a/Include/compile.h b/Include/compile.h
index 45854e9..deb8ee8 100644
--- a/Include/compile.h
+++ b/Include/compile.h
@@ -57,6 +57,8 @@
 } PyFutureFeatures;
 
 DL_IMPORT(PyFutureFeatures *) PyNode_Future(struct _node *, char *);
+DL_IMPORT(PyCodeObject *) PyNode_CompileFlags(struct _node *, char *,
+					      PyCompilerFlags *);
 
 #define NESTED_SCOPES_DEFAULT 0
 #define FUTURE_NESTED_SCOPES "nested_scopes"