[WIP] Replace deprecated MSBuild CreateItem with ItemGroup

Partial follow up to #733.

- This doesn't solve the "let's be nice with libgit2 binaries on non Windows platforms" issue.

- This only focus on trying to get a little bit more compatible when it comes to `.targets` files and the different flavors of Mono.

- This actually doesn't work. yet. (or at least I hope that it will *eventually* work)

Related:
 - https://bugzilla.xamarin.com/show_bug.cgi?id=3055
 - https://connect.microsoft.com/VisualStudio/feedback/details/615290/itemgroup-tag-works-different-to-deprecated-createitem-task