title: R devtools fails as "Package libxml-2.0 was not found in the pkg-config search path"
body: <p>I am trying to install devtools in R version 3.2.1, however when I do the following error is thrown:</p>
  
  <pre><code>Package libxml-2.0 was not found in the pkg-config search path.
  Perhaps you should add the directory containing libxml-2.0.pc
  to the PKG_CONFIG_PATH environment variable
  </code></pre>
  
  <p>when I run <code>dpkg -L libxml2-dev</code> in a terminal I find:</p>
  
  <pre><code>/usr
  /usr/bin
  /usr/bin/xml2-config
  /usr/share
  /usr/share/aclocal
  /usr/share/aclocal/libxml2.m4
  /usr/share/doc
  /usr/share/doc/libxml2-dev
  /usr/share/doc/libxml2-dev/copyright
  /usr/share/man
  /usr/share/man/man1
  /usr/share/man/man1/xml2-config.1.gz
  /usr/share/man/man3
  /usr/share/man/man3/libxml.3.gz
  /usr/lib
  /usr/lib/x86_64-linux-gnu
  /usr/lib/x86_64-linux-gnu/libxml2.a
  /usr/lib/x86_64-linux-gnu/pkgconfig
  /usr/lib/x86_64-linux-gnu/pkgconfig/libxml-2.0.pc
  /usr/lib/x86_64-linux-gnu/xml2Conf.sh
  /usr/include
  /usr/include/libxml2
  /usr/include/libxml2/libxml
  /usr/include/libxml2/libxml/globals.h
  /usr/include/libxml2/libxml/schematron.h
  /usr/include/libxml2/libxml/xlink.h
  /usr/include/libxml2/libxml/HTMLparser.h
  /usr/include/libxml2/libxml/pattern.h
  /usr/include/libxml2/libxml/xmlexports.h
  /usr/include/libxml2/libxml/xmlschemas.h
  /usr/include/libxml2/libxml/list.h
  /usr/include/libxml2/libxml/entities.h
  /usr/include/libxml2/libxml/xmlstring.h
  /usr/include/libxml2/libxml/encoding.h
  /usr/include/libxml2/libxml/hash.h
  /usr/include/libxml2/libxml/xmlmemory.h
  /usr/include/libxml2/libxml/relaxng.h
  /usr/include/libxml2/libxml/xmlsave.h
  /usr/include/libxml2/libxml/SAX2.h
  /usr/include/libxml2/libxml/xmlIO.h
  /usr/include/libxml2/libxml/xmlschemastypes.h
  /usr/include/libxml2/libxml/xpathInternals.h
  /usr/include/libxml2/libxml/schemasInternals.h
  /usr/include/libxml2/libxml/xmlmodule.h
  /usr/include/libxml2/libxml/valid.h
  /usr/include/libxml2/libxml/c14n.h
  /usr/include/libxml2/libxml/xmlwriter.h
  /usr/include/libxml2/libxml/tree.h
  /usr/include/libxml2/libxml/xmlunicode.h
  /usr/include/libxml2/libxml/nanohttp.h
  /usr/include/libxml2/libxml/catalog.h
  /usr/include/libxml2/libxml/xmlerror.h
  /usr/include/libxml2/libxml/nanoftp.h
  /usr/include/libxml2/libxml/xmlautomata.h
  /usr/include/libxml2/libxml/xinclude.h
  /usr/include/libxml2/libxml/HTMLtree.h
  /usr/include/libxml2/libxml/chvalid.h
  /usr/include/libxml2/libxml/parserInternals.h
  /usr/include/libxml2/libxml/xpointer.h
  /usr/include/libxml2/libxml/xmlversion.h
  /usr/include/libxml2/libxml/dict.h
  /usr/include/libxml2/libxml/xmlregexp.h
  /usr/include/libxml2/libxml/DOCBparser.h
  /usr/include/libxml2/libxml/parser.h
  /usr/include/libxml2/libxml/xmlreader.h
  /usr/include/libxml2/libxml/SAX.h
  /usr/include/libxml2/libxml/threads.h
  /usr/include/libxml2/libxml/debugXML.h
  /usr/include/libxml2/libxml/xpath.h
  /usr/include/libxml2/libxml/uri.h
  /usr/share/doc/libxml2-dev/README
  /usr/share/doc/libxml2-dev/NEWS.gz
  /usr/share/doc/libxml2-dev/AUTHORS
  /usr/share/doc/libxml2-dev/TODO.gz
  /usr/share/doc/libxml2-dev/changelog.Debian.gz
  /usr/lib/x86_64-linux-gnu/libxml2.so
  </code></pre>
  
  <p>to try and add this to the <code>PKG_CONFIG_PATH</code> I tried: <code>env PKG_CONFIG_PATH=/usr/lib/x86_64-linux-gnu/pkgconfig</code> however this does not seem to work. </p>