Previously the install target was having no effect because it ended up
calling the default target in the subdir Makefile instead of the
install target.

Resolve this by tying the tools/ocaml Makefiles into the generic
handling done by tools/Rules.mk.

Other changes arising in one way or another from this:
- Add libs/xl/META.in
- Update .hgignore for META files
- Create leading directories
- Remove existing module before installation in install targer
  (worksaround what appears to be a quirk of "ocamlfind install")
- Use the globally defined $(DESTDIR)
- Move "ocamlfind printfconf destdir" to a common variable,
  repurposing exising unused OCAMLDESTDIR, incorporating $(DESTDIR) at
  the same time.
- Drop a few unused variabe definitions (mainly to avoid deciding if
  $(DESTDIR) made sense for them or not.
- Pass -destdir to ocamlfind in uninstall target for symmetry with
  install target.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>