Emit clear error when using --layout=system with --build-type=complete.

[SVN r55733]

Jamroot

@@ -295,6 +295,17 @@ if ! $(layout)
 }
 layout-$(layout) = true ;
 
+if $(layout) = system && $(build-type) = complete
+{
+    ECHO "error: Cannot use --layout=system with --build-type complete." ;
+    ECHO "error: Please used either --layout=versioned or --layout=tagged " ;
+    ECHO "error: if you wish to build multiple variants." ;
+    if ! [ modules.peek : NT ] 
+    {        
+        ECHO "error: Note that --layout=system is default on Unix starting with Boost 1.40." ;
+    }    
+    EXIT ;
+}
 
 # Possible stage only location.
 local stage-locate = [ MATCH "^--stagedir=(.*)" : [ modules.peek : ARGV ] ] ;