If libjpeg-turbo is configured with a non-default prefix, such as /usr, then use...

If libjpeg-turbo is configured with a non-default prefix, such as /usr, then use the docdir variable defined by autoconf 2.60 and later, if available.  This will, for instance, install the documentation under /usr/share/doc/libjpeg-turbo by default if prefix=/usr, unless docdir is overridden.  When using earlier versions of autoconf, docdir is set to ${datadir}/doc, as it always has been.

git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/branches/1.4.x@1535 632fc199-4ca6-4c93-a231-07263d6284db