correctly if the compiler optimizes too clever.

This has happaned on HP-UX 11.23 (IA64) at optimization
level "+O2", causing bug#42213:
   Check for "stack overrun" doesn't work, server crashes

Fix it by adding a pragma that prevents this optimization.
As a result, it should be safe to use "+O2" on this platform
(unless there is some other, optimizer-related, bug which
is just currently masked because we use resudec optimization).