16.2.12 Standard library headers

Newer C++ implementations provide a new set of standard library header files. These are distinguished from older incompatible header files by their filenames--the new headers omit the conventional `.h' extension. Classes and other declarations in the new headers are placed in the std namespace. Detecting the kind of header files present on any given system is an ideal application of Autoconf. For instance, the header `<vector>' declares the class std::vector<T>. However, if it is not available, `<vector.h>' declares the class vector<T> in the global namespace.