It is said here http://www.joomlawebserver.com/booklibrary/booklibrary-1.0.x-free-manuals/19.-php5-extensions-for-booklibrary.html
PHP5 includes the XSL extension by default and it can be enabled by adding the argument --with-xsl[=DIR] to your configure line. DIR is the libxslt installation directory.
But I don't understand what configure line is about (is it php.ini ?)