You must #includeCyberTiggyr/giggle/giggle.hxx. You might have to tell your compiler about include directories with the -Iincludedir command line option to make this work, but this is the official recommended way to #include the header files for Giggle.
If you look in giggle.hxx, you'll see that it #includes some other header files in Giggle, & you might consider #includeing them directly yourself. Don't do that. the official header file interface to Giggle is #includeCyberTiggyr/giggle/giggle.hxx. The names, locations, or even existences of any other header files might change without notice.9.3
All the classes & functions in Giggle are in the namespace CyberTiggyr::giggle.