ocamlgraph: fix binary location (binaries had double prefix path)

Due to setting `DESTDIR` *and* `exec_prefix` (defaulted to `$prefix`), binaries
ended up in `$out/$out/bin` instead of just $out/bin. Not setting `DESTDIR` and adapting
the `LIBDIR` patch a little fixes this issue.

pkgs/development/ocaml-modules/ocamlgraph/default.nix

···
 
   buildInputs = [ ocaml findlib ocamlPackages.lablgtk ocamlPackages.camlp4 ];
 
-  patches = ./destdir.patch;
-
   # some patching is required so that the lablgtk2 library is taken into account. It
   # does not reside in a subdirectory of the default library path, hence:
   # * configure looked in the wrong path
   # * ocaml needs that directory and the stubs directory as -I flag
   postPatch = ''
-    sed -i 's@$(DESTDIR)$(OCAMLLIB)/ocamlgraph@$(DESTDIR)/lib/ocaml/${ocaml.version}/site-lib/ocamlgraph@' Makefile.in
+    sed -i 's@$(DESTDIR)$(OCAMLLIB)/ocamlgraph@$(prefix)/lib/ocaml/${ocaml.version}/site-lib/ocamlgraph@' Makefile.in
     sed -i 's@$OCAMLLIB/lablgtk2@${ocamlPackages.lablgtk}/lib/ocaml/${ocaml.version}/site-lib/lablgtk2@' configure Makefile.in
     sed -i 's@+lablgtk2@${ocamlPackages.lablgtk}/lib/ocaml/${ocaml.version}/site-lib/lablgtk2 -I ${ocamlPackages.lablgtk}/lib/ocaml/${ocaml.version}/site-lib/stublibs@' configure Makefile.in editor/Makefile
   '';