[win32] Add build scripts

[win32] Add build scripts

This patch adds a couple of scripts that can be used for
building Ceph for Windows along with its dependencies.

For now, we're going to cross compile it using mingw.
It's supposed to run on Ubuntu (feel free to update
the command that's installing some of the dependencies
within win32_deps_build.sh if you're interested in other
distros).

This commit also adds a short readme, describing the focus of
the Windows porting effort, the building process and the current
status.

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>
Signed-off-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>