I just found out that the GNU Hurd does not provide the 'SA_SIGINFO' constant, although it supplies the 'sigaction' function. There is nothing in the manual of sigaction ($ man sigaction) to indicate this. In fact the manual explains the usage of 'sigaction' with 'SA_SIGINFO'.

Apparently 'SA_SIGINFO' is a POSIX XSI extension and one should check its availability at compile time.

Thanks to the #hurd folks, especially Richard Braun (nick: syn) and Guillem Jover (nick: braindmg) for their help.