smarter PDF_VIEWER defaults, based on hints by Lars Noschinski;

--- a/etc/settings	Wed Apr 25 15:14:57 2012 +0200
+++ b/etc/settings	Wed Apr 25 15:44:26 2012 +0200
@@ -140,11 +140,14 @@
 
 # The pdf file viewer
 case "$ISABELLE_PLATFORM" in
+  *-linux)
+    PDF_VIEWER="xdg-open"
+    ;;
   *-darwin)
     PDF_VIEWER="open -W -n"
     ;;
-  *)
-    PDF_VIEWER=evince
+  *-cygwin)
+    PDF_VIEWER="cygstart"
     ;;
 esac