Add isnan() and isinf() to the global namespace in libstdc++'s <cmath>.

The standard (n3242, section 17.6.1.1, paragraph 4) says that, because these are
declared as macros in the C specification (ev

Add isnan() and isinf() to the global namespace in libstdc++'s <cmath>.

The standard (n3242, section 17.6.1.1, paragraph 4) says that, because these are
declared as macros in the C specification (even though they are
implemented as functions in the C++ library) they should be in the global
namespace.

A surprising number of configure checks rely on this. It was broken by recent
cleanups to math.h.

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