Followed the precedent in SPI and Wire libraries, adding #ifdefs to...

Followed the precedent in SPI and Wire libraries, adding #ifdefs to conditionally declare Wire interface externs

libraries/Wire/Wire.h

@@ -106,11 +106,23 @@ class TwoWire : public Stream
     //static const uint32_t XMIT_TIMEOUT = 100000;
 };
 
-extern TwoWire Wire;
-extern TwoWire Wire1;
-extern TwoWire Wire2;
-extern TwoWire Wire3;
-extern TwoWire Wire4;
-extern TwoWire Wire5;
+#if WIRE_INTERFACES_COUNT > 0
+  extern TwoWire Wire;
+#endif
+#if WIRE_INTERFACES_COUNT > 1
+  extern TwoWire Wire1;
+#endif
+#if WIRE_INTERFACES_COUNT > 2
+  extern TwoWire Wire2;
+#endif
+#if WIRE_INTERFACES_COUNT > 3
+  extern TwoWire Wire3;
+#endif
+#if WIRE_INTERFACES_COUNT > 4
+  extern TwoWire Wire4;
+#endif
+#if WIRE_INTERFACES_COUNT > 5
+  extern TwoWire Wire5;
+#endif
 
 #endif