I just tried to reproduce your problem by installing everything from the wixel-dev-bundle-121027.exe (except Notepad++ because I already had it) onto my Windows 8 64-bit machine. When I ran "make VERBOSE=true" everything built fine. It worked in the normal Windows Command Prompt and also in another shell, Git Bash.
The two error messages you are seeing make me think that SDCC is trying to invoke its preprocessor utility but that utility is not found. SDCC is not really handling the error correctly and thinks that the preprocessor just had an empty output, so it is warning that ISO C forbids an empty source file.
If you just want to start on your project, the quickest solution is probably to uninstall SDCC and then reinstall it in a path with no spaces (e.g. C:/SDCC).
However, if I were in your situation I would want to make a nice bug report to the SDCC developers so they can fix it. I have attached a minimal SDCC project that builds without any errors on my machine. It also uses the --verbose option so we can get more information. It has two sdcc invocations: a simple one and one more like what is happening in the Wixel SDK. You can get it here:
test_sdcc1.zip (343 Bytes)
If you download it, extract it, and run build.bat in a Command Prompt, does it work for you? Do both SDCC invocations work?
What happens if you use the latest SDCC release from their sourceforge download page (3.3.0)? It probably doesn't matter if you install the 64-bit or the 32-bit version. What happens if you install the latest SDCC snapshot build?