POSIX math: Define my_fpclassify only if no c99_classify already.

("c99_classify already" = we saw HAS_FPCLASSIFY and fpclassify macro,
 which together smell like C99)
POSIX math: erf implementation from http://www.johndcook.com/

(Also add sources to other formulas where they weren't obvious.)
POSIX math: better hypot.
POSIX math: isless etc tests.
POSIX math: more hypot tests.
POSIX math: looser error bound for erf test.
POSIX math: erfc implementation.

(Chiani, Dardari & Simon (2003), via Wikipedia)
POSIX math: add isinf/isnan/isfinite tests.
POSIX math: add tests.