Introduce isnanf() as an alias for isnanf(), and make the isnan()
macro expand to isnanf() instead of isnanf() for float arguments.
This change is needed because isnanf() isn't declared in strict POSIX
or C99 mode.

Compatibility note: Apps using isnan(float) that are compiled after
this change won't link against an older libm.

Reported by: Florian Forster <octo@verplant.org>