GNU make manual 翻译( 一百零一)

   By declaring the subdirectories as phony targets (you must do this as
the subdirectory obviously always exists; otherwise it won't be built)
you can remove these problems:

     SUBDIRS = foo bar baz

     .PHONY: subdirs $(SUBDIRS)

     subdirs: $(SUBDIRS)

     $(SUBDIRS):
             $(MAKE) -C $@

     foo: baz

   Here we've also declared that the `foo' subdirectory cannot be built
until after the `baz' subdirectory is complete; this kind of
relationship declaration is particularly important when attempting
parallel builds.