Wire: Define WIRE_HAS_TIMEOUT

In commit deea929 (Introduce non compulsory Wire timeout), some
timeout-related functions were added. To allow writing portable
sketches, it is important for those sketch to know whether they are
using a Wire library version that is new enough to offer these functions
without having to rely on version number checking (since other Arduino
cores have their own versioning schemes).

This adds a WIRE_HAS_TIMEOUT macro, similar to the existing WIRE_HAS_END
macro, to facilitate that.

This relates to arduino#42.

libraries/Wire/src/Wire.h

@@ -30,6 +30,9 @@
 
 // WIRE_HAS_END means Wire has end()
 #define WIRE_HAS_END 1
+// WIRE_HAS_TIMEOUT means Wire has setWireTimeout(), getWireTimeoutFlag
+// and clearWireTimeoutFlag()
+#define WIRE_HAS_TIMEOUT 1
 
 class TwoWire : public Stream
 {