Much like SERVER-8842, we should have an SCons option that sets the minimum supported windows version for the build. Setting this option would select a value for _WIN32_WINNT and pass that as a define on the build line, rather than our current practice of hard coding _WIN32_WINNT in targetver.h

As part of this change, the --win2008plus flag should be removed, and the MONGO_USE_SRW macro should be replaced with a check of _WIN32_WINNT against the windows version definitions found here: http://msdn.microsoft.com/en-us/library/windows/desktop/aa383745(v=vs.85).aspx