title: can we undefine/unset a variable in makefile
body: <p>Team,</p>
  
  <p>I have an environment variable which i would like to undefined/unset during the  compilation of that particular project and then enable that back..</p>
  
  <p>i have tried using.. the below code..</p>
  
  <p><code>ifdef ${ENV_VAR_TEST}
      undefine ${ENV_VAR_TEST}
      endif
  </code></p>
  
  <p>but i still see that its get reflected in the compilation environment... are these correct steps to undefined/unset a variable ?..i am i missing some basics here..</p>
  
  <p>Thanks for your time .</p>