configure.ac: Define _XOPEN_SOURCE to 500 on HP-UX.
configure.ac: Define _XOPEN_SOURCE to 500 on HP-UX.

On HP-UX, we need -D_XOPEN_SOURCE=500 so that Berkeley socket API
functions use 'socklen_t's for sockaddr struct lengths instead of
ints.

Signed-off-by: Lennert Buytenhek <8858498612e3a522354216649776e3032dcb7e5b@wantstofly.org>

buytenh/ivykis,buytenh/ivykis