Fix the invocation to ocamlfind for syntax extensions

Need to:
1/ call ocamlfind to get the `link-byte` flags
2/ call ocamlfind to get the `pp-byte` flags
3/ do not call `%d/%a` as some META files wrongly encode external archive
   dependencies in their archive field
Merge pull request #119 from samoht/ocamlfind

Fix the invocation to ocamlfind for syntax extensions