is defined and if so, what its value is, are
C++ is not identical to ISO C.
Mandating that __STDC__
be defined would require that translators make an incorrect claim.
Each implementation must choose the behavior that will be most
useful to its marketplace.
Effect on original feature:
Change to semantics of well-defined feature.
Difficulty of converting:
How widely used:
Programs and headers that reference __STDC__