cppfiles.c (stack_include_file): Harmonize system headerness tests.

cppfiles.c (stack_include_file): Harmonize system headerness tests.

	* cppfiles.c (stack_include_file): Harmonize system headerness tests.
	* cppfiles.c (stack_include_file): Only stack a file if there
	is something to do.  Return a boolean indicating whether a
	buffer was stacked or not.
	(_cpp_execute_include): Similarly.
	(_cpp_read_file): Similarly.
	* cpphash.h (_cpp_read_file, _cpp_execute_include): Update prototypes.

From-SVN: r45083