https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95989
--- Comment #11 from Florian Weimer <fw at gcc dot gnu.org> --- It turns out that libc.a did not contain pthread_self until glibc 2.27. The symbol was only present in libc.so.6 (as a weird forwarder, for compatibility with long-defunct LinuxThreads). This means there could be some fallout from requiring a definition of pthread_self in the statically linked case, in the form of linking failures without -lpthread on older glibc (such 2.17, as including in Red Hat Enterprise Linux 7).
