Transforming a project setting

gistfile1.scala

source:

  lazy val sxr = scalaz.configure{
    p =>
      Project(
        id=p.id+"-sxr",
        base = file("."),
        settings = p.settings ++
          Seq(
            addCompilerPlugin("org.scala-sbt.sxr" % "sxr_2.10" % "0.3.0-SNAPSHOT"),
            scalacOptions <+= scalaSource in Compile map { _ => "-P:sxr:base-directory:" + file(".").getAbsolutePath }
          )
      )
  }

Notes: 
  scalaz:Project
  Project.configure : Project => Project
  
Current workaround: p.asInstanceOf[ProjectDefinition[_]].settings ...

  Error:
  /scalaz/project/build.scala:171: ambiguous reference to overloaded definition,
[error] both method settings in trait Project of type (ss: sbt.Def.Setting[_]*)sbt.Project
[error] and  method settings in trait ProjectDefinition of type => Seq[sbt.Def.Setting[_]]
[error] match expected type ?
[error]         settings = p.settings ++
[error]                      ^

Which is line 8 here.