include cleanups: remove unused headers and add feature test macros

src/thread/pthread_attr_init.c

@@ -1,5 +1,4 @@
 #include "pthread_impl.h"
-#include <string.h>
 
 int pthread_attr_init(pthread_attr_t *a)
 {

src/thread/pthread_sigmask.c

@@ -1,6 +1,5 @@
 #include <signal.h>
 #include <errno.h>
-#include <pthread.h>
 #include "syscall.h"
 
 int pthread_sigmask(int how, const sigset_t *restrict set, sigset_t *restrict old)

src/thread/synccall.c

@@ -1,6 +1,5 @@
 #include "pthread_impl.h"
 #include <semaphore.h>
-#include <string.h>
 
 static struct chain {
 	struct chain *next;