contrib/buildsystems: add a backend for modern Visual Studio versions

contrib/buildsystems: add a backend for modern Visual Studio versions

Based on the previous patches in this patch series that fixed the
generator for `.vcproj` files (which were used by Visual Studio prior to
2015 to define projects), this patch offers to generate project
definitions for neweer versions of Visual Studio (which use `.vcxproj`
files).

To that end, this patch copy-edits the generator of the `.vcproj`.

In addition, we now use the `vcpkg` system which allows us to build
Git's dependencies (e.g. curl, libexpat) conveniently. The support
scripts were introduced in the `jh/msvc` patch series, and with this
patch we initialize the `vcpkg` conditionally, in the `libgit` project's
`PreBuildEvent`. To allow for parallel building of the projects, we
therefore put `libgit` at the bottom of the project hierarchy.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>