adventuregamestudio/ags-manual

AI Summary: Modify the `configure.ac` script for the AGS documentation build system to accept a `--with-source` parameter. This parameter will specify the directory containing the markdown source files, allowing for building from different source directories (e.g., `source-ags3`, `source-ags4`). The change must also address the existing hardcoded `source/` directory within the build directory.