In SafeAssert.cpp define fsync when unavailable

Summary: Posix does not require fsync to be provided, so define it as a local empty function when is it is not available

Reviewed By: yfeldblum

Differential Revision: D27243532

fbshipit-source-id: e7a9c31312e288458a9dc11736b238a378671caf

folly/lang/SafeAssert.cpp

@@ -498,6 +498,14 @@ constexpr int stderr_fileno = STDERR_FILENO;
 
 #endif
 
+#if !defined(_WIN32) && !defined(_POSIX_FSYNC)
+
+int fsync(int fh) {
+  return 0;
+}
+
+#endif
+
 void writeStderr(const char* s, size_t len) {
   fileutil_detail::wrapFull(write, stderr_fileno, const_cast<char*>(s), len);
 }