Each version of SUS specifies a different value for _POSIX_VERSION,
_POSIX2_VERSION, and _XOPEN_VERSION. glibc also changes the value
of the other _POSIX2_ variables but not the _POSIX_* variables.
_POSIX_TIMERS should be set to a version number, not just 1.
The _POSIX_V7_* macros were missing, which was not noticed because
the V6 values were aliased in sysconf (<unistd.h>).
Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>