build: run commands silently on config targets

And also add an "error: " prefix, to make the output clearer.

Before:

    $ rm -f config.mk; make config.mk
    printf 'run ./configure to generate %s\n' "config.mk" >&2
    run ./configure to generate config.mk
    false
    make: *** No rule to make target 'config.mk'.  Stop.

After:

    $ rm -f config.mk; make config.mk
    error: run ./configure to generate config.mk
    make: *** No rule to make target 'config.mk'.  Stop.

This amends commit e21637c ("makefiles: add generated files as
dependencies", 2022-06-23) / PR netblue30#5219.

Makefile

@@ -25,8 +25,8 @@ ALL_ITEMS = $(APPS) $(SBOX_APPS) $(SBOX_APPS_NON_DUMPABLE) $(MYLIBS)
 all: all_items mydirs $(MAN_TARGET) filters
 
 config.mk config.sh:
-	printf 'run ./configure to generate %s\n' "$@" >&2
-	false
+	@printf 'error: run ./configure to generate %s\n' "$@" >&2
+	@false
 
 .PHONY: all_items $(ALL_ITEMS)
 all_items: $(ALL_ITEMS)