* 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
