adding ISABELLE_GHC environment setting to mira configuration isabelle makeall all on lxbroy10

--- a/Admin/mira.py	Thu Jun 09 10:43:42 2011 +0200
+++ b/Admin/mira.py	Thu Jun 09 11:57:39 2011 +0200
@@ -191,6 +191,8 @@
 ML_HOME="/home/polyml/polyml-svn/x86_64-linux"
 ML_SYSTEM="polyml-5.4.1"
 ML_OPTIONS="-H 8000 --gcthreads 6"
+
+ISABELLE_GHC="/usr/bin/ghc"
 '''
 
     isabelle_home = paths[0]