title: Using Makefile, how can I delete object files in a sub-directory?
body: <p>I am new to using makefile in Windows, and I've been trying hard to learn this.
  When I try to delete object files using clean option, it keeps making errors.
  There's Makefile and 3 sub-directories and each sub-directory has a .cpp file. I want to delete .o files in each sub-directory, but when I write down my code like
  clean:
          delete *.o
  it doesn't work.
  How can I delete object files in sub-directories?</p>