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.
Add constant for PPP-Max-PayLoad tag.

Submitted by:	Dmitry Luhtionov <dmitryluhtionov gmail.com>