accept x86_64 results gracefully -- NB: Mac OS does report that if booted in 64 bit mode;

--- a/lib/scripts/isabelle-platform	Tue Apr 20 11:26:25 2010 +0200
+++ b/lib/scripts/isabelle-platform	Tue Apr 20 11:31:14 2010 +0200
@@ -26,6 +26,10 @@
           ISABELLE_PLATFORM64=x86_64-darwin
         fi
         ;;
+      x86_64)
+        ISABELLE_PLATFORM=x86-darwin
+        ISABELLE_PLATFORM64=x86_64-darwin
+        ;;
       Power* | power* | ppc)
         ISABELLE_PLATFORM=ppc-darwin
         ;;
@@ -33,7 +37,7 @@
     ;;
   CYGWIN_NT*)
     case $(uname -m) in
-      i?86)
+      i?86 | x86_64)
         ISABELLE_PLATFORM=x86-cygwin
         ;;
     esac
@@ -45,7 +49,7 @@
           sparc)
             ISABELLE_PLATFORM=sparc-solaris
             ;;
-          i?86)
+          i?86 | x86_64)
             ISABELLE_PLATFORM=x86-solaris
             ;;
         esac
@@ -54,7 +58,7 @@
     ;;
   FreeBSD|NetBSD)
     case $(uname -m) in
-      i?86)
+      i?86 | x86_64)
         ISABELLE_PLATFORM=x86-bsd
         ;;
     esac