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Without this change, 'configure' fails because the recently-added
wait3 prototype in config.h messes up later 'configure' tests.
Fix this problem by droping wait3 and WRETCODE, as they're
no longer needed on hosts that are current porting targets.
* configure.ac (wait3, WRETCODE): Remove, fixing a FIXME.
All uses changed to waitpid and WEXITSTATUS.
* src/syswait.h (WRETCODE): Remove, consistently with ../configure.ac.