standards/112180: tgamma on negative integers returns Inf instead of NaN

[Bruce Evans]

On Fri, 27 Apr 2007, Vincent Lefèvre wrote:

> Though POSIX allows an implementation-defined value to be returned, this is mainly in the case where NaN is not supported, and NaN is the best choice and is what tgamma returns on other platforms (Linux, HP-UX, Mac OS X).

Also, Annex F of C99 (about the IEEE-754/IEC-mumble binding) requires a NaN
plus the "invalid" exception.

Bruce