title: Using preprocessor defined during compilation time
body: <p>I want to define a preprocessor in compile time so that I can use it like this in the code</p>
  
  <pre><code>#ifdef MYFLAG
  do some thing
  #endif
  </code></pre>
  
  <p>I added that flag using AC_DEFINE([MYFLAG], [1], [My Flag]) in configure.ac and ran autoreconf. I can see it defined in configure file but still I can not use it.  </p>