[issue25720] Fix curses module compilation with ncurses6

[Masayuki Yamamoto]

Masayuki Yamamoto added the comment:

Hi, I finished various things and tackle the issue again, I opened PR 1689 at last.

Changes from previous patch:
* If ncurses doesn't have both is_pad function and _flags field of WINDOW, NCURSES_OPAQUE is defined as zero to make WINDOW to non-opaque type before including ncurses.h.
* The conditional compile on function definition are kept, it replaces WINDOW_HAS_FLAGS with py_is_pad macro itself.
* Unindent the blocks that places after section of the conditional compile on function definition.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue25720>
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