Constants defined on http://www.opengroup.org/onlinepubs/009695399/basedefs/unistd.h.html and http://www.opengroup.org/onlinepubs/009695399/functions/sysconf.html, but not defined in libraries:
_SC_AIO_LISTIO_MAX
_SC_AIO_MAX
_SC_AIO_PRIO_DELTA_MAX
_SC_BC_BASE_MAX
_SC_BC_DIM_MAX
_SC_BC_SCALE_MAX
_SC_BC_STRING_MAX
_SC_COLL_WEIGHTS_MAX
_SC_DELAYTIMER_MAX
_SC_EXPR_NEST_MAX
_SC_HOST_NAME_MAX
_SC_LINE_MAX
_SC_LOGIN_NAME_MAX
_SC_MQ_OPEN_MAX
_SC_MQ_PRIO_MAX
_SC_ADVISORY_INFO
_SC_BARRIERS
_SC_ASYNCHRONOUS_IO
_SC_CLOCK_SELECTION
_SC_CPUTIME
_SC_FSYNC
_SC_IPV6
_SC_MEMLOCK
_SC_MEMLOCK_RANGE
_SC_MESSAGE_PASSING
_SC_MONOTONIC_CLOCK
_SC_PRIORITIZED_IO
_SC_PRIORITY_SCHEDULING
_SC_RAW_SOCKETS
_SC_READER_WRITER_LOCKS
_SC_REGEXP
_SC_SHARED_MEMORY_OBJECTS
_SC_SHELL
_SC_SPAWN
_SC_SPIN_LOCKS
_SC_SPORADIC_SERVER
_SC_SS_REPL_MAX
_SC_SYNCHRONIZED_IO
_SC_THREAD_CPUTIME
_SC_THREAD_PRIO_INHERIT
_SC_THREAD_PRIO_PROTECT
_SC_THREAD_SAFE_FUNCTIONS
_SC_THREAD_SPORADIC_SERVER
_SC_TIMEOUTS
_SC_TIMERS
_SC_TRACE
_SC_TRACE_EVENT_FILTER
_SC_TRACE_EVENT_NAME_MAX
_SC_TRACE_INHERIT
_SC_TRACE_LOG
_SC_TRACE_NAME_MAX
_SC_TRACE_SYS_MAX
_SC_TRACE_USER_EVENT_MAX
_SC_TYPED_MEMORY_OBJECTS
_SC_V6_ILP32_OFF32
_SC_V6_ILP32_OFFBIG
_SC_V6_LP64_OFF64
_SC_V6_LPBIG_OFFBIG
_SC_2_C_BIND
_SC_2_C_DEV
_SC_2_CHAR_TERM
_SC_2_FORT_DEV
_SC_2_FORT_RUN
_SC_2_LOCALEDEF
_SC_2_PBS
_SC_2_PBS_ACCOUNTING
_SC_2_PBS_CHECKPOINT
_SC_2_PBS_LOCATE
_SC_2_PBS_MESSAGE
_SC_2_PBS_TRACK
_SC_2_SW_DEV
_SC_2_UPE
_SC_2_VERSION
_SC_THREAD_DESTRUCTOR_ITERATIONS
_SC_THREAD_KEYS_MAX
_SC_THREAD_THREADS_MAX
_SC_RE_DUP_MAX
_SC_RTSIG_MAX
_SC_SIGQUEUE_MAX
_SC_SYMLOOP_MAX
_SC_TIMER_MAX
_SC_TTY_NAME_MAX
_SC_XBS5_ILP32_OFF32 (LEGACY)    //macros..?
_SC_XBS5_ILP32_OFFBIG (LEGACY)
_SC_XBS5_LP64_OFF64 (LEGACY)
_SC_XBS5_LPBIG_OFFBIG (LEGACY)
_SC_XOPEN_CRYPT
_SC_XOPEN_ENH_I18N
_SC_XOPEN_LEGACY
_SC_XOPEN_REALTIME
_SC_XOPEN_REALTIME_THREADS
_SC_XOPEN_SHM
_SC_XOPEN_STREAMS
_SC_XOPEN_UNIX
_SC_XOPEN_VERSION