preprocessor: Update mkdeps for modules

This is slightly different to the original patch I posted.  This adds
separate module target and dependency functions (rather than a single
bi-modal function).

	libcpp/
	* include/cpplib.h (struct cpp_options): Add modules to
	dep-options.
	* include/mkdeps.h (deps_add_module_target): Declare.
	(deps_add_module_dep): Declare.
	* mkdeps.c (class mkdeps): Add modules, module_name, cmi_name,
	is_header_unit fields.  Adjust cdtors.
	(deps_add_module_target, deps_add_module_dep): New.
	(make_write): Write module dependencies, if enabled.

libcpp/include/cpplib.h

@@ -528,6 +528,9 @@ struct cpp_options
        one.  */
     bool phony_targets;
 
+    /* Generate dependency info for modules.  */
+    bool modules;
+
     /* If true, no dependency is generated on the main file.  */
     bool ignore_main_file;