Prevent setting TOP in the env from upsetting BB into an infinite loop.

[SVN r28774]

v1/allyourbase.jam

@@ -526,6 +526,12 @@ DEPENDS all shell files lib dll exe obj test : first ;
 NOTFILE all first shell files lib dll exe obj dirs clean uninstall test nothing ;
 ALWAYS clean uninstall ;
 
+#
+# Prevent external variables from interfeering with some esnetial intenal
+# variables.
+#
+TOP = ;
+
 #
 # Rules
 #