Undefined use of weak symbols in gnulib

[Paul Eggert]

I'm still not quite following, but the message I'm getting is "don't 
mess with dynamic detection of whether the pthread functions are linked 
in, because there's no way to do it reliably." If so, I suppose Gnulib 
will have to require pthread to be linked into all libraries that use 
any of these functions, which has some negative consequences (at least 
for glibc 2.33 and earlier).

Bruno's the expert here, though; I hope he has time to weigh in.