The #include directive allows external
header files to be processed by the compiler.
Syntax:
#include <header-file>
or
#include "source-file"
When enclosing the file with < and >, then the implementation
searches the known header directories for the file (which is
implementation-defined) and processes it. When enclosed with
double quotation marks, then the entire contents of the
source-file is replaced at this point. The searching manner for
the file is implementation-specific.
