In order to build Soup, just follow the generic directions in the INSTALL file.

All changes to Soup must be approved by the maintainers before they
can be commited. Mail your patch (created using diff -u) with a
detailed ChangeLog and a description of changes to
soup-list@ximian.com [1], and do not apply it until it is approved by
one of the maintainers.

Please be sure to follow the GNOME Programming guidelines for all code
submitted. The programming guidelines can be found in the white-papers
CVS module.

[1] To subscribe to the Soup discussion list, send mail with the word
    "Subscribe" in the message body to soup-list-request@ximian.com.