With clang-16, implicit function definitions are treated as errors. As a
result, the check for pthread_getname_np fails because
pthread_getname_np is only provided under _GNU_SOURCE (see
pthread_getname_np(3))

> Checking if "pthread_getname_np" : links: NO

The compilation failure is

> error: implicit declaration of function 'pthread_getname_np' [-Werror,-Wimplicit-function-declaration]

The inclusion of pthread.h lib/rb-debug.c must be moved above the system
headers because they may include pthread.h themselves (and in practice
unistd.h does). If that change is not done, lib/rb-debug.c will fail to
compile for the same reason as the configure test.

Note that to test this, one must disable prctl() detection.

Bug: https://bugs.gentoo.org/898926