Comment 5 for bug 1687059

       --build=type
              Specifies the build type from a comma-separated list of
              components (since dpkg 1.18.5). Passed to dpkg-genchanges.

              The allowed values are:

              source Builds the source package. Note: when using this value
                     standalone and if what you want is simply to (re-)build
                     the source package, using dpkg-source is always better as
                     it does not require any build dependencies to be
                     installed to be able to call the clean target.

Interesting, --build=source is the same -S which is what build-source passes down to dpkg-buildpackage. I'm not sure how that last sentence is possible given the description of this bug?