Jenkins: configurable clean build

--- a/src/Pure/Admin/ci_profile.scala	Tue Oct 18 12:01:54 2016 +0200
+++ b/src/Pure/Admin/ci_profile.scala	Tue Oct 18 15:57:34 2016 +0200
@@ -22,7 +22,7 @@
       Build.build_selection(
         options = options,
         progress = progress,
-        clean_build = true,
+        clean_build = clean,
         verbose = true,
         max_jobs = jobs,
         dirs = include,
@@ -136,6 +136,7 @@
   /* profile */
 
   def documents: Boolean = true
+  def clean: Boolean = true
 
   def threads: Int
   def jobs: Int