add winrt detection

include/libtorrent/config.hpp

@@ -275,6 +275,14 @@ POSSIBILITY OF SUCH DAMAGE.
 #define TORRENT_USE_PREADV 1
 #define TORRENT_USE_PWRITEV 1
 
+// ==== WINRT ===
+#if defined(WINAPI_FAMILY_PARTITION)
+# if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP) \
+  && !WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_DESKTOP)
+#  define TORRENT_WINRT
+# endif
+#endif
+
 // ==== SOLARIS ===
 #elif defined sun || defined __sun
 #define TORRENT_SOLARIS