c-pch.c: Add comments in various places.

c-pch.c: Add comments in various places.

	* c-pch.c: Add comments in various places.
	(struct c_pch_validity): Add the lengths of various strings.
	(host_machine): New static.
	(target_machine): New static.
	(get_ident): Bump version number.
	(pch_init): Write out version, host, target validity data.
	(c_common_valid_pch): Check version, host, target.
	* Makefile.in (c-pch.o): Add version.h; define HOST_MACHINE and
	TARGET_MACHINE.

From-SVN: r72774