add stdc-predef.h for library-level predefined macros

modern compilers (for gcc, versions 4.8 and later) automatically
pre-include <stdc-predef.h> to obtain the values of certain predefined
macros specified by ISO C but which reflect properties of the library
implementation, not just the compiler. provide values indicating that
wchar_t is Unicode-encoded and that Annex F (IEEE floating point) is
supported unless the compiler indicates otherwise.

based on patch by Masanori Ogino.

include/stdc-predef.h

+#ifndef _STDC_PREDEF_H
+#define _STDC_PREDEF_H
+
+#define __STDC_ISO_10646__ 201206L
+
+#if !defined(__GCC_IEC_559) || __GCC_IEC_559 > 0
+#define __STDC_IEC_559__ 1
+#endif
+
+#endif