Drop the together keyword on the Clean action

Since the PM merge we now have way too many targets,
here jam clean just fails in execve() due to too long arguments.

jam clean is now very verbose but at least it works.

Another option could be to override the Clean rule itself
to clean in batch by splitting the list...
Patches welcome.

build/jam/OverriddenJamRules

@@ -5,7 +5,7 @@ actions Chmod1
 }
 
 # Overridden to allow spaces in file names.
-actions piecemeal together existing Clean
+actions piecemeal existing Clean
 {
 	$(RM) "$(>)"
 }