allow modification of REPOS_NAME and REPOS from outside

--- a/Admin/makedist	Wed Mar 14 00:34:56 2012 +0100
+++ b/Admin/makedist	Wed Mar 14 12:20:42 2012 +0100
@@ -4,10 +4,9 @@
 
 ## global settings
 
-REPOS_NAME="isabelle"
-REPOS="http://isabelle.in.tum.de/repos/${REPOS_NAME}"
-
-DISTPREFIX=${DISTPREFIX:-~/tmp/isadist}
+REPOS_NAME="${REPOS_NAME:-isabelle}"
+REPOS="${REPOS:-http://isabelle.in.tum.de/repos/${REPOS_NAME}"
+DISTPREFIX="${DISTPREFIX:-~/tmp/isadist}"
 
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