Raise system requirement on Windows to Vista/2008

We need the newer versions of Windows to get access to better
thread primitives.

CMakeLists.txt

   set(BUILD_WINVNC 1)
 endif()
 
-# Minimum version is Windows XP SP2 (5.2)
+# Minimum version is Windows Vista/2008 (6.0)
 if(WIN32)
-  add_definitions(-D_WIN32_IE=0x0502 -D_WIN32_WINNT=0x0502)
+  add_definitions(-D_WIN32_WINNT=0x0600)
 endif()
 
 if(CMAKE_SIZEOF_VOID_P MATCHES 8)