When compile an Arduino Library, it fails to compile with the message:

error: WConstants.h: No such file or directory

The solution is to delete the line

#include "WConstants.h"

and then in the .h file, add the following:

#if ARDUINO >= 100
 #include "Arduino.h"
#else
 #include "WProgram.h"
#endif

This way, the code will compile, and it will be compatible to Arduino 0023 and 1.0 versions.