Remove __ from HAVE_EXTERN_ENVIRON_DECLARED__

I think that the trailing __ got added in false analogy to
HAVE_MACRO__func__, HAVE_MACRO__FUNC__, and HAVE_MACRO__FUNCTION__.
But those macros actually indicate the presence of __func__,
__FUNC__, and __FUNCTION__ respectively.  The __ at the end of
HAVE_EXTERN_ENVIRON_DECLARED would only be appropriate if the
environ were declared__, whatever that means.

(As a side-note, HAVE_MACRO__func__ and so on should probably be
renamed HAVE_MACRO___func__ and so on.  But that can wait.)

This is an identifier renaming only.

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