title: How to set `Java headers gen.:` for R (to install `rJava`)
body: <p>I am trying to install the package <code>rJava</code> in R on Arch Linux.</p>
  
  <p>I got the error:</p>
  
  <pre><code>configure: error: One or more Java configuration variables are not set.
  Make sure R is configured with full Java support (including JDK). Run
  R CMD javareconf as root to add Java support to R.
  </code></pre>
  
  <p>Running <code>sudo R CMD javareconf</code>, I can see that <code>Java headers gen.:</code> is not set, but I have failed to figure out how to set it.</p>
  
  <p>Here is the full java configuration seen by R (beside <code>Java haders gen.</code>, the rest looks ok to me):</p>
  
  <pre><code>Java interpreter : /usr/bin/java
  Java version     : 10
  Java home path   : /usr/lib/jvm/java-10-openjdk
  Java compiler    : /usr/bin/javac
  Java headers gen.: 
  Java archive tool: /usr/bin/jar
  </code></pre>
  
  <p>I do have <code>javah</code> in <code>/usr/bin</code>. But how do I "tell" this to R?</p>