Fix PTHREAD_THREADS_MAX.

We don't have a compile-time limit on the number of threads,
and we don't have a definite run-time limit either.

Bug: http://b/27617302
Change-Id: I6a6fe083e7b655d24eb9e7ef7f3e0280d483080b
diff --git a/libc/include/sys/limits.h b/libc/include/sys/limits.h
index be79cf3..f84253e 100644
--- a/libc/include/sys/limits.h
+++ b/libc/include/sys/limits.h
@@ -120,8 +120,11 @@
 #define  _XOPEN_VERSION             700       /* by Posix definition */
 
 
-#define PTHREAD_DESTRUCTOR_ITERATIONS 4     // >= _POSIX_THREAD_DESTRUCTOR_ITERATIONS
-#define PTHREAD_KEYS_MAX              128   // >= _POSIX_THREAD_KEYS_MAX
-#define PTHREAD_THREADS_MAX           2048  // bionic has no specific limit
+/* >= _POSIX_THREAD_DESTRUCTOR_ITERATIONS */
+#define PTHREAD_DESTRUCTOR_ITERATIONS 4
+/* >= _POSIX_THREAD_KEYS_MAX */
+#define PTHREAD_KEYS_MAX 128
+/* bionic has no specific limit */
+#undef PTHREAD_THREADS_MAX
 
 #endif