libxls - Read XLS files from C

This is libxls, a C library for reading Excel files in the nasty old
binary OLE format, plus a command-line tool for converting XLS to CSV
(named, appropriately enough, xls2csv).

Note: The command-line tool is, instead, named libxls2csv by this
SlackBuild script to avoid conflict with xls2csv tool from catdoc
package.