build: Define _GNU_SOURCE for pthread_getname_np
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.