unix: guard use of PTHREAD_STACK_MIN

PTHREAD_STACK_MIN is an optional part of the POSIX spec and
NetBSD deliberately does not implement it as it's a variable
value in runtime.

PR-URL: https://github.com/libuv/libuv/pull/2252
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

src/unix/thread.c

@@ -219,8 +219,10 @@ int uv_thread_create_ex(uv_thread_t* tid,
     pagesize = (size_t)getpagesize();
     /* Round up to the nearest page boundary. */
     stack_size = (stack_size + pagesize - 1) &~ (pagesize - 1);
+#ifdef PTHREAD_STACK_MIN
     if (stack_size < PTHREAD_STACK_MIN)
       stack_size = PTHREAD_STACK_MIN;
+#endif
   }
 
   if (stack_size > 0) {