simplify pthread_atfork detection in AtFork

Differential Revision: D34224951

fbshipit-source-id: f3f4fe2866c795a285382d59be07f5d7f0fb6cfa

folly/detail/AtFork.cpp

@@ -111,21 +111,15 @@ class AtForkList {
 
  private:
   AtForkList() {
-#if FOLLY_HAVE_PTHREAD_ATFORK
-    int ret = pthread_atfork(
+    int ret = 0;
+#if FOLLY_HAVE_PTHREAD_ATFORK // if no pthread_atfork, probably no fork either
+    ret = pthread_atfork(
         &AtForkList::prepare, &AtForkList::parent, &AtForkList::child);
+#endif
     if (ret != 0) {
       throw_exception<std::system_error>(
           ret, std::generic_category(), "pthread_atfork failed");
     }
-#elif !__ANDROID__ && !defined(_MSC_VER)
-// pthread_atfork is not part of the Android NDK at least as of n9d. If
-// something is trying to call native fork() directly at all with Android's
-// process management model, this is probably the least of the problems.
-//
-// But otherwise, this is a problem.
-#warning pthread_atfork unavailable
-#endif
   }
 };
 } // namespace