src: Use fcntl and FD_CLOEXEC if O_CLOEXEC is undefined

We may run into race condition if execve is called at the same time
when fcntl is called.  But we just does this for now to compile
nghttp2 applications under older kernel.

src/util.cc

@@ -539,10 +539,21 @@ int reopen_log_file(const char *path)
     fd = open(path, O_WRONLY | O_APPEND | O_CREAT | O_CLOEXEC,
               S_IRUSR | S_IWUSR | S_IRGRP);
   }
-#else // !__ANDROID__ && !ANDROID
+#elif defined O_CLOEXEC
+
   auto fd = open(path, O_WRONLY | O_APPEND | O_CREAT | O_CLOEXEC,
                  S_IRUSR | S_IWUSR | S_IRGRP);
-#endif // !__ANDROID__ && !ANDROID
+#else // !O_CLOEXEC
+
+  auto fd = open(path, O_WRONLY | O_APPEND | O_CREAT,
+                 S_IRUSR | S_IWUSR | S_IRGRP);
+
+  // We get race condition if execve is called at the same time.
+  if(fd != -1) {
+    fcntl(fd, F_SETFD, FD_CLOEXEC);
+  }
+
+#endif // !O_CLOEXEC
 
   if(fd == -1) {
     return -1;