Makefile: define grouped target rule correctly

A po4a command will generate all the $(PO_FILES) or $(MXML_FILES),
instead of generating them one by one.  Use '&:' provided by GNU Make >=
4.3 to define the grouped target correctly.

I don't want to spend too much time to find a "portable" solution.

Makefile

@@ -10,7 +10,7 @@ PO_FILES = $(patsubst $(LFS_EN)/%.xml, $(MLANG)/%.po, $(XML_FILES))
 
 -include local.mk
 
-$(PO_FILES): $(XML_FILES) mkpo4acfg.py changelogtranslator.py
+$(PO_FILES) &: $(XML_FILES) mkpo4acfg.py changelogtranslator.py
 	mkdir -pv $(POT_DIRS)
 	$(CMD_FIND_XML) | ./mkpo4acfg.py > po4a.cfg
 	po4a --no-translations po4a.cfg
@@ -49,7 +49,7 @@ ORIG_FILES = $(MLANG)/book/general.ent.orig \
 booksrc: $(MBOOK_FILES) $(ORIG_FILES)
 	[ ! -e $(MLANG)/fix.sh ] || (pushd $(MLANG)/book; sh ../fix.sh; popd)
 
-$(MXML_FILES): $(XML_FILES) $(PO_FILES) mkpo4acfg.py
+$(MXML_FILES) &: $(XML_FILES) $(PO_FILES) mkpo4acfg.py
 	mkdir -pv $(POT_DIRS)
 	$(CMD_FIND_XML) | ./mkpo4acfg.py > po4a.cfg
 	po4a po4a.cfg

README.md

@@ -20,6 +20,7 @@ contribute other translations, please mail to `[email protected]`
 
 首先安装依赖项：
 
+* GNU Make (>= 4.3)
 * [po4a](https://po4a.org/) (>= 0.62)
 * [polib](https://pypi.org/project/polib/)
 * [libxml2](https://www.linuxfromscratch.org/blfs/view/svn/general/libxml2.html)