World Wide Web Consortium (W3C) Question:

If you use a library (like libwww) and include it in the source code, or compile/link to it, you must also include the copyright license. However, if you merely provide a compile option you are not obligated to include our license. Any subsequent party that links or includes against the library is obligated to include the license. (If you include the option, you might do your users a favor by pointing to it yourself.) To restate our policy: it is the responsibility of the person who causes our software to be included in subsequent distributions (either in source, object, or executable code) to abide by our terms.