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. This will allow easier
porting of software that's taking advantage of the automatic enabling
since it's not now possible at all.
PR: 255290
Sponsored by: Netflix