Add a simple atan2() test.

(Not similar to the 'torture tests' that were removed.)
apidoc for Perl_isinfnan.
Configure: add i_stdint.
HAS_C99 was oddly defined, and oddly used.

It was defined as "C version high enough, but not VMS",
but used as "does it have intmax_t".  Now that we have
I_STDINT, we can do better.
Another way to detect C99.