libarchive: Define _XOPEN_SOURCE=500 on HP-UX
The HP-UX <wchar.h> header provides 'mbstate_t' in C89/C90 mode only if _XOPEN_SOURCE is defined to exactly 500. Type 'mbstate_t' was introduced in C89/C90 Normative Amendment 1, aka C94/C95, adding support international character sets. It is part of C99 but not C89/C90.
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