Use a more bulletproof test for whether finite() and isinf() are present.

Use a more bulletproof test for whether finite() and isinf() are present.
It seems that recent gcc versions can optimize away calls to these functions
even when the functions do not exist on the platform, resulting in a bogus
positive result.  Avoid this by using a non-constant argument and ensuring
that the function result is not simply discarded.  Per report from
François Laupretre.