title: gfortran error : isnan has no implicit type
body: <p>I am using intrinsic <a href="https://gcc.gnu.org/onlinedocs/gcc-4.7.3/gfortran/ISNAN.html" rel="nofollow noreferrer">isnan</a> to check <code>NaN</code> which may occur in my program under certain situation.</p>
  <p>It was perfectly working in my earlier machine, now after I have switched to a new machine, I am finding the following compilation error message:</p>
  <pre><code>Error: Function 'isnan' at (1) has no IMPLICIT type
  </code></pre>
  <p>However, the code gets compiled with Intel Fortran (ifort) compiler.</p>
  <p>The version of the gfortran in the present machine is the following:</p>
  <pre><code>GNU Fortran (GCC) 4.1.2 20080704 (Red Hat 4.1.2-55)
  </code></pre>
  <p>How can I proceed?</p>