build(cmake): fix for boost 1.89

Starting with boost 1.89, system is header only. This causes configure errors in Simple-Web-Server.

See:

• https://github.com/boostorg/filesystem/issues/336#issuecomment-3010301949
• https://github.com/boostorg/system/commit/7a495bb46d7ccd808e4be2a6589260839b0fd3a3