winhttp: Fix build on Mac OS X 10.5.8.

dlls/winhttp/session.c

@@ -1297,7 +1297,7 @@ static WCHAR *build_wpad_url( const char *hostname, const struct addrinfo *ai )
 
 static BOOL get_system_proxy_autoconfig_url( char *buf, DWORD buflen )
 {
-#ifdef HAVE_CORESERVICES_CORESERVICES_H
+#if defined(MAC_OS_X_VERSION_10_6) && MAC_OS_X_VERSION_MAX_ALLOWED >= MAC_OS_X_VERSION_10_6
     CFDictionaryRef settings = CFNetworkCopySystemProxySettings();
     const void *ref;
     BOOL ret = FALSE;