win32: don't for _WIN32_WINNT to Win2K

If the system allows newer API's we should downversion it.

caca/driver/win32.c

@@ -19,7 +19,9 @@
 
 #if defined(USE_WIN32)
 
+#if !defined(_WIN32_WINNT) || _WIN32_WINNT < 0x500 /* _WIN32_WINNT_WIN2K */
 #define _WIN32_WINNT 0x500 /* Require WinXP or later */
+#endif
 #define WIN32_LEAN_AND_MEAN
 #include <windows.h>