title: how to describe in Makefile to compile c to obj only if the target is missing?
body: <p>I 've a library in my project and I want to force gmake to build object files for this library only if the library file is not already build.</p>
  
  <p>Currently, following the dependencies, the objects are always build.  </p>
  
  <p>So is it possible to describe such rules that will prevent compiling, if the library already exist?    </p>
  
  <p>added example:</p>
  
  <pre><code>all: prj.exe
      @echo build all.
  
  prj.exe: main.o libbar.a
      gcc main.o -lbar -L. -o prj.exe
  
  libbar.a: foo.o
      ar rcs libbar.a foo.o
  
  %.o: %.c
      gcc -c $&lt; -o $@
  </code></pre>