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

chardi/musl,chardi/musl,chardi/musl