* cpp.texi: Document #pragma GCC dependency

	* cpp.texi: Document #pragma GCC dependency
	* cppfiles.c (open_include_file): Set date to unknown.
	(_cpp_compare_file_date): New function.
	(read_include_file): Set file date.
	* cpphash.h (struct include_file): Add date member.
	(_cpp_compare_file_date): Prototype.
	* cpplib.c (parse_include): Add trail parameter. Adjust.
	(do_include): Adjust parse_include call.
	(do_import): Likewise.
	(do_include_next): Likewise.
	(gcc_pragmas): Add dependency pragma.
	(do_pragma_dependancy): New pragma.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@34808 138bc75d-0d04-0410-961f-82ee72b054a4