title: "Non-zero exit status" error downloading XML and RCurl R packages
body: <p><strong>I'm having this problem:</strong></p>
  
  <pre><code>&gt; install.packages("RCurl")
  Installing package(s) into ‘/home/username/R/x86_64-redhat-linux-gnu-library/2.13’
  (as ‘lib’ is unspecified)
  trying URL 'http://lib.stat.cmu.edu/R/CRAN/src/contrib/RCurl_1.95-3.tar.gz'
  Content type 'application/x-gzip' length 868491 bytes (848 Kb)
  opened URL
  ==================================================
  downloaded 848 Kb
  
  * installing *source* package ‘RCurl’ ...
  checking for curl-config... no
  Cannot find curl-config
  ERROR: configuration failed for package ‘RCurl’
  * removing ‘/home/username/R/x86_64-redhat-linux-gnu-library/2.13/RCurl’
  Warning in install.packages :
    installation of package 'RCurl' had non-zero exit status
  
  The downloaded packages are in
      ‘/tmp/RtmpKyAgF8/downloaded_packages’
  </code></pre>
  
  <p><strong>AND:</strong></p>
  
  <pre><code>&gt; install.packages("XML")
  Installing package(s) into ‘/home/username/R/x86_64-redhat-linux-gnu-library/2.13’
  (as ‘lib’ is unspecified)
  trying URL 'http://lib.stat.cmu.edu/R/CRAN/src/contrib/XML_3.95-0.1.tar.gz'
  Content type 'application/x-gzip' length 1724282 bytes (1.6 Mb)
  opened URL
  ==================================================
  downloaded 1.6 Mb
  
  * installing *source* package ‘XML’ ...
  checking for gcc... gcc
  checking for C compiler default output file name... a.out
  checking whether the C compiler works... yes
  checking whether we are cross compiling... no
  checking for suffix of executables... 
  checking for suffix of object files... o
  checking whether we are using the GNU C compiler... yes
  checking whether gcc accepts -g... yes
  checking for gcc option to accept ISO C89... none needed
  checking how to run the C preprocessor... gcc -E
  No ability to remove finalizers on externalptr objects in this verison of R
  checking for sed... /bin/sed
  checking for pkg-config... /usr/bin/pkg-config
  checking for xml2-config... no
  Cannot find xml2-config
  ERROR: configuration failed for package ‘XML’
  * removing ‘/home/username/R/x86_64-redhat-linux-gnu-library/2.13/XML’
  Warning in install.packages :
    installation of package 'XML' had non-zero exit status
  
  The downloaded packages are in
      ‘/tmp/RtmpKyAgF8/downloaded_packages’
  </code></pre>