Default to Windows 7 SDK

Some of our current crypto bindings require the Win7 SDK.
Once we can rework those, this could reasonably be changed
to Win Vista or possibly Win Server 2003.

I'd like to completely remove support for WinXP and earlier.

CMakeLists.txt

@@ -204,7 +204,7 @@ OPTION(ENABLE_INSTALL "Enable installing of libraries" ON)
 
 SET(POSIX_REGEX_LIB "AUTO" CACHE STRING "Choose what library should provide POSIX regular expression support")
 SET(ENABLE_SAFESEH "AUTO" CACHE STRING "Enable use of /SAFESEH linker flag (MSVC only)")
-SET(WINDOWS_VERSION "" CACHE STRING "Set Windows version to use (Windows only)")
+SET(WINDOWS_VERSION "WIN7" CACHE STRING "Set Windows version to use (Windows only)")
 
 IF(ENABLE_COVERAGE)
 	include(LibarchiveCodeCoverage)