Bump Windows requirement to XP SP2

That's even more generous than Microsoft themselves, and we need
such a new version for basic IPv6 support.

CMakeLists.txt

@@ -80,16 +80,9 @@ if(NOT DEFINED BUILD_WINVNC)
   set(BUILD_WINVNC 1)
 endif()
 
-# Minimum version is Windows 2000 (5.0)
+# Minimum version is Windows XP SP2 (5.2)
 if(WIN32)
-  if(NOT CMAKE_SIZEOF_VOID_P MATCHES 8)
-    add_definitions(-D_WIN32_IE=0x0500 -D_WIN32_WINNT=0x0500)
-  else()
-    set(WIN64 1)
-    # Win64 doesn't like us requesting a Windows version that didn't have
-    # 64-bit support. Request XP (5.1) instead.
-    add_definitions(-D_WIN32_IE=0x0501 -D_WIN32_WINNT=0x0501)
-  endif()
+  add_definitions(-D_WIN32_IE=0x0502 -D_WIN32_WINNT=0x0502)
 endif()
 
 if(CMAKE_SIZEOF_VOID_P MATCHES 8)