Allow the secret to be safely specified externally

[kohsuke]

The way the code is written today expects users to modify the file that's under the version control system. This is inherently dangerous as a developer needs to locally edit this file and then be very careful not to accidentally commit the change in to the repository.

A better way to do this is to either allow these constants to be defined through a compiler switch (for interactive dekstop development) or in a separate file (outside the ForcePad source tree and the -I option allows you to have this import statement resolve the header file correctly.)

This will also work better in a build server, where an in-place modification to a file is tricky.