Use nanosleep instead of usleep.

usleep is guarded by feature macro insanity. Use nanosleep which looks to be
less unfriendly.

Change-Id: I75cb2284f26cdedabb19871610761ec7440b6ad3
Reviewed-on: https://boringssl-review.googlesource.com/7710
Reviewed-by: Emily Stark (Dunn) <estark@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/thread_test.c

@@ -53,7 +53,8 @@ static int wait_for_thread(thread_t thread) {
 #else
 
 #include <pthread.h>
-#include <unistd.h>
+#include <string.h>
+#include <time.h>
 
 typedef pthread_t thread_t;
 
@@ -84,7 +85,10 @@ static void once_init(void) {
 #if defined(OPENSSL_WINDOWS)
   Sleep(1 /* milliseconds */);
 #else
-  usleep(1000 /* microseconds */);
+  struct timespec req;
+  memset(&req, 0, sizeof(req));
+  req.tv_nsec = 1000000;
+  nanosleep(&req, NULL);
 #endif
 }