When _ISOC11_SOURCES is defined for glibc at the same time
__POSIX_C_SOURCE is defined, it extends the __POSIX_C_SOURCE definition
by exaclty what C11 adds to the spec for each of the affected headers.
It goes one step further by doing this by default when C11 or higher
compliation mode is selected.
There's a diversity of opinion about whether this automatic extension is
standards conformant, so only enable C11 visibility when specifically
requested since the Open Group standard allows this when other feature
macros are defined by other standards. Should POSIX clarify this in
the future, it would be easy enough to correct.
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