title: R portable: not able to install some packages
body: <p>I have the Rportable 3.3.1 version. Some packages (such as ggplot2) were installed without problem. But some packages that must be compiled from binaries cannot be installed. I have Rtools version 3.4.0.1962 installed on my computer but it seems that it is not detected by Rportable.
  The packages that create a problem are:</p>
  
  <ul>
  <li>httpuv</li>
  <li>jsonlite</li>
  <li>sourcetools</li>
  </ul>
  
  <p>Which all return the following error:</p>
  
  <pre><code>&gt;install.packages("jsonlite")
  Package which is only available in source form, and may need compilation of C/C++/Fortran: ‘jsonlite’
  These will not be installed
  </code></pre>
  
  <p>How can I tell Rportable where to locate Rtools to compile packages source?</p>