Reland: Don't expose unavailable cstdio functions.

Marked unsupported for C++03 and C++11 since this test uses alias
declarations, and at least one C++03 bot was failing with
-Wc++11-extensions.

Change-Id: I8c3a579edd7eb83e0bc74e85d116b68f22400161
Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 75c4408653753fbb8e273ad41cd41997d498e7d3
diff --git a/include/cstdio b/include/cstdio
index 0f3f42d..d0492a0 100644
--- a/include/cstdio
+++ b/include/cstdio
@@ -131,9 +131,13 @@
 using ::ungetc;
 using ::fread;
 using ::fwrite;
+#ifndef _LIBCPP_HAS_NO_FGETPOS_FSETPOS
 using ::fgetpos;
+#endif
 using ::fseek;
+#ifndef _LIBCPP_HAS_NO_FGETPOS_FSETPOS
 using ::fsetpos;
+#endif
 using ::ftell;
 using ::rewind;
 using ::clearerr;