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.