Define _HPUX_ALT_XOPEN_SOCKET_API where necessary

There is a nasty bug in HP-UX: When using the proper socket APIs that
take a socklen_t (which is 64 bit in LP64 mode), it still calls into
methods that expect an int - so there is a type mismatch between the
headers and the compiled system libraries. This leads to all socklen_t
being interpreted as 0 (due to big endian). Defining
_HPUX_ALT_XOPEN_SOCKET_API makes it use the correct symbols in the
system libraries instead.