Actually, "the make fails" are some of the most dreaded words in the world of software. They mean that somewhere in the system one piece expects to find another piece, but doesn't.
A quick look at the XMBC instructions doesn't say anything terribly helpful, aside from a truly frightening list of config options and file munging. A quick look at the recommended OS version Pisces didn't give much information either, except that it's a "more complete" installation of Debian.
Usually a make will fail with a list of errors explaining the immediate issue that caused the problem. Do you happen to remember what those were?
FWIW, if you haven't deleted the code, you should be able to reproduce the failure pretty quickly. One of the key features in 'make' is its ability to decide which files need to be recompiled and which don't. It creates a list of all the things needed to build each piece (called a 'topologically sorted list') and then checks to see how old the files are. If the build product is newer than any file in its dependency chain, 'make' assumes that piece of the build is still good. If one of the dependencies is newer than something that needs it, 'make' assumes that sequence of parts needs to be rebuilt.
So.. if the files are still there and you haven't edited, moved, or 'touch'ed anything (a command that changes the file's last-changed date to "now"), typing `make` should skip straight to the faliure without crunching along for eight hours first.
When you void a product warrany, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.