工作目录下辖，test6 test9目录，各有一个makefile文件

试图实现make -C dir 怎么不行呢？
主makefile
    1   sources=test9 test6
 2  subi:forces
 3  $(foreach dir,$(sources),$(MAKE) -C $(dir))
 4  forces:;
 5  .PHONY:clean
 6  clean:
                           $(RM) /test6/main
 make失败：
    make -C test9 make -C test6
make[1]: *** test6: No such file or directory.  Stop.
makefile:3: recipe for target 'subi' failed
make: *** [subi] Error 2