don't use system wide installed ocaml packages

Index: configure.ac
--- configure.ac.orig
+++ configure.ac
@@ -280,7 +280,6 @@ AS_IF([test "x${enable_certificate_check}" = "xno"], [
 
 AC_CHECK_PROGS(FETCH,[curl wget],no)
 
-AC_CHECK_TOOL(DUNE,dune)
 AC_CHECK_TOOL(PATCH,patch)
 AC_CHECK_TOOL(BUNZIP2,bunzip2)
 
