title: error while installing gmp (frama-c prerequisite) on mac os catalina using opam on terminal
body: <p>I ran these commands to install Frama-C as said on <a href="http://frama-c.com/install-sulfur-20171101.html#installing-frama-c-on-mac-os-x" rel="nofollow noreferrer">http://frama-c.com/install-sulfur-20171101.html#installing-frama-c-on-mac-os-x</a>:</p>
  <blockquote>
  <ol>
  <li><code>brew install autoconf opam</code></li>
  </ol>
  <p>Do not forget to <code>opam init</code> and <code>eval $(opam config env)</code> for a proper OPAM installation (if not already done before on your machine).</p>
  <ol start="2">
  <li><p><code>brew install gmp gtk+ gtksourceview libgnomecanvas</code></p>
  </li>
  <li><p><code>brew install graphviz</code></p>
  <p><code>opam install altgr-ergo why3</code></p>
  </li>
  </ol>
  </blockquote>
  <p>I encountered an error after the 3rd command (of course there are 2 more commands to install frama-c completely, but the same error is showing up after the last command too).</p>
  <pre><code>ERROR while compiling conf-gmp.1
  
  test.c:7:2: error: implicit declaration of function '__gmp_init' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
           __gmp_init();
  </code></pre>