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When you say "I compile a short test program that includes
only unistd.h" did you literally mean a program that only
includes unistd.h (i.e. has no main function)? If not, what
is the short program? In particular, make sure it includes
the line

void *p = setgroups;

Please locate the declaration of setgroups in unistd.h (or
whereever it is declared), and report whether this is in a
conditional compilation section, and if so, what precisely
the condition is.