build-sys: make man/systemd.directives.xml depend on man/custom-entit…

…ies.ent

Currently, the following command sequence fails:

  make distclean
  ./autogen.sh c
  make distcheck

That's because the command invoked to build man/systemd.directives.xml needs
man/custom-entities.ent to function, which itself isn't a dependency.

The $(filter-out $<,$^) logic used to filter out everything from the
prerequisites except for the first word, which doesn't work anymore
now. Use $(SOURCE_XML_FILES) instead.

Makefile.am

@@ -714,9 +714,9 @@ man/systemd.index.xml: $(top_srcdir)/tools/make-man-index.py $(NON_INDEX_XML_FIL
 	$(AM_V_at)$(MKDIR_P) $(dir $@)
 	$(AM_V_GEN)$(PYTHON) $< $@ $(filter-out $<,$^)
 
-man/systemd.directives.xml: $(top_srcdir)/tools/make-directive-index.py $(SOURCE_XML_FILES)
+man/systemd.directives.xml: $(top_srcdir)/tools/make-directive-index.py man/custom-entities.ent $(SOURCE_XML_FILES)
 	$(AM_V_at)$(MKDIR_P) $(dir $@)
-	$(AM_V_GEN)$(PYTHON) $< $@ $(filter-out $<,$^)
+	$(AM_V_GEN)$(PYTHON) $< $@ $(SOURCE_XML_FILES)
 
 CLEANFILES += \
 	man/systemd.index.xml \