Whenever an unqualified name is used
in the specification of a declaration D
in [support] through [thread] or [depr],
its meaning is established
as-if by performing unqualified name lookup ([basic.lookup.unqual])
in the context of D.
The C standard library headers ([depr.c.headers]) also define
names within the global namespace, while the C++ headers for C library
facilities ([headers]) can also define names within the global namespace.