win32: fix Visual Studio 2010 build with WINVER >= 0x600

If no WINVER and/or _WIN32_IWNNT define was set, the Windows platform
SDK often defaults to high value, e.g. 0x601 (whoch may probably depend
on the Windows version being used, in my case Windows 7).

If WINVER >= 0x600 then winsock2.h includes some defines for WSAPoll(),
e.g. POLLIN, POLLPRI, POLLOUT etc. These defines clash with cURL's
lib/select.h.

Make sure HAVE_STRUCT_POLLFD is defined then.

Bug: http://curl.haxx.se/bug/view.cgi?id=1282
Reported-by: "kdekker"
Patch-by: Marcel Raad

lib/config-win32.h

@@ -390,21 +390,6 @@
 #  define SIZEOF_SIZE_T 4
 #endif
 
-/* ---------------------------------------------------------------- */
-/*                          STRUCT RELATED                          */
-/* ---------------------------------------------------------------- */
-
-/* Define if you have struct sockaddr_storage. */
-#if !defined(__SALFORDC__) && !defined(__BORLANDC__)
-#define HAVE_STRUCT_SOCKADDR_STORAGE 1
-#endif
-
-/* Define if you have struct timeval. */
-#define HAVE_STRUCT_TIMEVAL 1
-
-/* Define if struct sockaddr_in6 has the sin6_scope_id member. */
-#define HAVE_SOCKADDR_IN6_SIN6_SCOPE_ID 1
-
 /* ---------------------------------------------------------------- */
 /*               BSD-style lwIP TCP/IP stack SPECIFIC               */
 /* ---------------------------------------------------------------- */
@@ -572,6 +557,25 @@
 #  endif
 #endif
 
+/* ---------------------------------------------------------------- */
+/*                          STRUCT RELATED                          */
+/* ---------------------------------------------------------------- */
+
+/* Define if you have struct sockaddr_storage. */
+#if !defined(__SALFORDC__) && !defined(__BORLANDC__)
+#define HAVE_STRUCT_SOCKADDR_STORAGE 1
+#endif
+
+/* Define if you have struct timeval. */
+#define HAVE_STRUCT_TIMEVAL 1
+
+/* Define if struct sockaddr_in6 has the sin6_scope_id member. */
+#define HAVE_SOCKADDR_IN6_SIN6_SCOPE_ID 1
+
+#if HAVE_WINSOCK2_H && defined(_WIN32_WINNT) && (_WIN32_WINNT >= 0x0600)
+#define HAVE_STRUCT_POLLFD 1
+#endif
+
 /* ---------------------------------------------------------------- */
 /*                        LARGE FILE SUPPORT                        */
 /* ---------------------------------------------------------------- */