Update sync_libgit2 target in Makefile to copy zlib files

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Add zlib files as built-in dependency

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Build zlib on Windows

* scripts/build_Makevars.r: Add OBJECTS.zlib to Makevars.win to build
  zlib on Windows. Add extra linebreak after comment "do not edit by
  hand" in Makevars.in and Makevars.win.

* Makevars.win: Updated

* Makevars.in: Updated

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Merge branch 'master' of github.com:ropensci/git2r