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Version of library

   There are four different versions of the math library routines: IEEE,
POSIX, X/Open, or SVID.  The version may be selected at runtime by
setting the global variable `_LIB_VERSION', defined in `math.h'.  It
may be set to one of the following constants defined in `math.h':
`_IEEE_', `_POSIX_', `_XOPEN_', or `_SVID_'.  The `_LIB_VERSION'
variable is not specific to any thread, and changing it will affect all
   The versions of the library differ only in how errors are handled.
   In IEEE mode, the `matherr' function is never called, no warning
messages are printed, and `errno' is never set.
   In POSIX mode, `errno' is set correctly, but the `matherr' function
is never called and no warning messages are printed.
   In X/Open mode, `errno' is set correctly, and `matherr' is called,
but warning message are not printed.
   In SVID mode, functions which overflow return
3.40282346638528860e+38, the maximum single precision floating point
value, rather than infinity.  Also, `errno' is set correctly, `matherr'
is called, and, if `matherr' returns 0, warning messages are printed
for some errors.  For example, by default `log(-1.0)' writes this
message on standard error output:
     log: DOMAIN error
   The library is set to X/Open mode by default.