windows: target Windows 7 and add support for more than 64 CPUs

windows: target Windows 7 and add support for more than 64 CPUs

Introduce support for targeting the build to either Windows XP or
Windows 7 (the default). When targeting Windows 7 this allows us to use
the process group APIs which are required are required to support more
than 64 CPUs.

These changes mean if you want a Windows binary that supports versions
of Windows below Windows 7/Windows Server 2008 R2 you will need to
explicitly use the --target-win-ver=xp configure option at build time.
Such builds will lack features (such as the ability to access CPUs
beyond those in fio's default process group) that are dependent on
recent Windows APIs.

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>