POSIX math: Add M_ constants like M_PI.
POSIX math: Implement glue for the C99 math functions.

Plus the Bessel functions j0, j1, jn, y0, y1, yn: common extensions
from BSD/SVID/XPG4.

Plus the constants INFINITY/NAN.
POSIX math: Add FP_ILOGB0 and FP_ILOGBNAN.
POSIX math: Configure scan for j0 and j0l.

(j0 is canary forthe  Bessel functions, common math.h extensions,
 not dependent on C99, j0l is its long double version.)
POSIX math: Hopefully better use of Configure symbols.
POSIX math: Add more C99 math tests.

(Note: these all will fail if we don't have C99 math.  Duh.)
POSIX math: Configure scan for acosh.
POSIX math: Skip the POSIX C99 math tests if no acosh().
POSIX math: Use HAS_ACOSH to enable/disable the C99 math.
POSIX math: Win32 shuffling.
POSIX math: Have only one not_here for each API.
POSIX math: HP-UX exceptions.
POSIX math: Portability emulations and constants.

Plus fix the cmp_ok tests which had epsilon 18 magnitudes off.
(Didn't cause any false positives, luckily.)
POSIX math: fpclassify emulation.
POSIX math: Configure scan d_fegetround and i_fenv.
POSIX math: More handling of legacy implementations.
POSIX math: Add fegetround() and fesetround().
POSIX math: lrint and rint emulations.
POSIX math: Configure scan fpgetround

Legacy BSD way of fegetround.
POSIX math: todo for emulating fegetround() with fpgetround().