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.

contrib/libstdc++/include/c_std/std_cmath.h

@@ -589,6 +589,8 @@ _GLIBCXX_BEGIN_NAMESPACE(std)
     { return ::__gnu_cxx::__capture_isunordered(__f1, __f2); }
 
 _GLIBCXX_END_NAMESPACE
+using std::isnan;
+using std::isinf;
 
 #endif /* _GLIBCXX_USE_C99_FP_MACROS_DYNAMIC */
 #endif