Add cpp #warning to instruct builder to ignore HP type-pun warning

When building on the HP-UX platform with GCC, the user will see a type-punned pointer warning in client_code.c at line 1490. This warning can be ignored, and this commit adds a #warning to inform the user of this fact, plus the documentation below as a comment in client_code.c itself.

While the "fd_set" type is defined in /usr/include/sys/_fd_macros.h as a struct of an array of <I>longs</I> in accordance with the XPG4 standard's requirements, the macros for the FD operations "pretend it is an array of int32_t's so the binary layout is the same for both Narrow and Wide processes," as described in _fd_macros.h. In the FD_SET, FD_CLR, and FD_ISSET macros at line 101, the result is cast to an "__fd_mask *" type, which is defined as int32_t at _fd_macros.h:82.

This conflict between the "long fds_bits[]" array in the XPG4-compliant fd_set structure, and the cast to an int32_t - not long - pointer in the macros, is what causes the type-pun warning. It can be ignored since it is just the HP-UX OS working as designed.