* conform/conformtest.pl: Implement optional-macro.
* conform/data/math.h-data: Update for XPG6.
* math/math.h (HUGE): Define as FLT_MAX value but don't use
FLT_MAX. Don't include <float.h>.
(MAXFLOAT): Likewise.
* math/math.h: Define MATH_ERRNO and MATH_ERREXCEPT.