9a778368f2 "move destdir creation to installPhase" moved directory
creation to preInstallHooks and exposed a bug in ocaml_libvirt:
```
ocaml-libvirt> ocamlfind install -ldconf ignore libvirt \
ocaml-libvirt> ../META *.so *.a *.cma *.cmx *.cmxa *.cmi *.mli
ocaml-libvirt> ocamlfind: Bad configuration: Cannot mkdir
/nix/store/...aml-libvirt-0.6.1.5/lib/ocaml/4.12.0/site-lib/libvirt
because a path component does not exist or is not a directory
ocaml-libvirt> make[1]: *** [Makefile:134: install-opt] Error 2
```
The change restore pre/post phases by overriding build targets instead
of overriding full phases.
1526d916ab