Added support for ``if __debug__:'' -- if -O is given, this form is

recognized by the code generator and code generation for the test and
the subsequent suite is suppressed.

One must write *exactly* ``if __debug__:'' or ``elif __debug__:'' --
no parentheses or operators must be present, or the optimization is
not carried through.  Whitespace doesn't matter.  Other uses of
__debug__ will find __debug__ defined as 0 or 1 in the __builtin__
module.