libxml2's threads.c redeclares the pthread functions with __attribute((weak)), on compilers that support it, presumably in order to avoid pulling in a dependency on libpthread.so if the application using libxml2 does not use threads. This usage of weak references is valid for shared libraries, assuming all of the pthread functions lie in a single shared library file, but it is incorrect and dangerous for use with static linking.

Suppose a program (to be static linked) uses threads but never calls pthread_equal. There will be no non-weak reference to pthread_equal, so it won't be linked in, and thus xmlInitThreads will wrongly consider the program non-threaded based on the condition (pthread_equal != NULL) failing.

Unfortunately I don't believe there's any correct way to "weak reference" pthreads like this when static linking. The best suggestion I can make is to disable the weak reference trick when building object files for the static library and only use it for the shared library. However at that point it might be preferable just to disable it entirely.