title: R plots some unicode characters but not others
body: <p>our sysadmin just upgraded our operating system to SLES12SP1. I reinstalled Rv3.2.3 and tried to make plots. I use <code>cairo_pdf</code> and try to make a plot with the x-label being <code>\u0298</code> i.e. the solar symbol, but it doesn't work: the label just comes out blank. For example:</p>
  
  <pre><code>cairo_pdf('Rplots.pdf')
  plot(1, xlab='\u0298') # the x-label comes up blank
  dev.off()
  </code></pre>
  
  <p>This used to work, but for some reason it does not anymore. It works with other characters, e.g. </p>
  
  <pre><code>cairo_pdf('Rplots.pdf')
  plot(1, xlab='\u2113') # the x-label comes up with the \ell symbol
  dev.off()
  </code></pre>
  
  <p>When I just paste in the solar symbol, i.e.</p>
  
  <pre><code>plot(1, xlab='ʘ')
  </code></pre>
  
  <p>then I get the warning</p>
  
  <pre><code>Warning messages:
  1: In title(...) :
    conversion failure on 'ʘ' in 'mbcsToSbcs': dot substituted for &lt;ca&gt;
  </code></pre>
  
  <p>The machine is German, but I am using the US English UTF-8 locale:</p>
  
  <pre><code>&gt; sessionInfo()
  R version 3.2.3 (2015-12-10)
  Platform: x86_64-pc-linux-gnu (64-bit)
  Running under: SUSE Linux Enterprise Server 12 SP1
  
  locale:
   [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
   [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8
   [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8
   [7] LC_PAPER=en_US.UTF-8       LC_NAME=C
   [9] LC_ADDRESS=C               LC_TELEPHONE=C
  [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C
  </code></pre>
  
  <p>Any tips on how I can get the solar symbol to appear? </p>