kbuild: Handle removed headers

New and old way to handle dependencies would choke when a file
#include'd by other files was removed, since the dependency on it was
still recorded, but since it was gone, make has no idea what to do about
it (and would complain with "No rule to make <file> ...")

We now add targets for all the previously included files, so make will
just ignore them if they disappear.

Rules.make

@@ -446,7 +446,7 @@ if_changed = $(if $(strip $? \
 # execute the command and also postprocess generated .d dependencies
 # file
 
-if_changed_dep = $(if $(strip $? \
+if_changed_dep = $(if $(strip $? $(filter-out FORCE $(wildcard $^),$^)\
 		          $(filter-out $(cmd_$(1)),$(cmd_$@))\
 			  $(filter-out $(cmd_$@),$(cmd_$(1)))),\
 	@set -e; \

scripts/fixdep.c

@@ -292,7 +292,7 @@ void parse_dep_file(void *map, size_t len)
 		exit(1);
 	}
 	memcpy(s, m, p-m); s[p-m] = 0;
-	printf("%s: \\\n", target);
+	printf("deps_%s := \\\n", target);
 	m = p+1;
 
 	clear_config();
@@ -314,7 +314,8 @@ void parse_dep_file(void *map, size_t len)
 		}
 		m = p + 1;
 	}
-	printf("\n");
+	printf("\n%s: $(deps_%s)\n\n", target, target);
+	printf("$(deps_%s):\n", target);
 }
 
 void print_deps(void)