* libc/include/sys/features.h: Add POSIX.1-permitted definition of
_POSIX_C_SOURCE if not already defined and _XOPEN_SOURCE has an
appropriate value. Specifically, check for POSIX.1-2008 & 2001 values.
* libc/include/sys/config.h: Add #include <sys/features.h> so that all
includes get the new check added to it.
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