title: How to compile f77 and f90 files together with gfortran
body: <p>I have a few dozens files with f77-functions and I would like to include them into my f90-program. How do I bring it altogether with gfortran? If I just try to</p>
  
  <pre><code>gfortran myprogram.f90
  </code></pre>
  
  <p>it complains for f77-code in the other files. As I understand, it would like to see all files put into definite standard (f77 of f90).</p>