DDE27 CC, AMU and Srcbuild hiding compile errors.

1. Given a missing function declaration CC with terminate with a non-zero return code, but still write a runnable output file.
2. AMU will then terminate with a non-zero return code, but does not delete the file. GNU Make has an option to do this, but AMU does not as far as I can tell.
3. Srcbuild will rerun AMU.
4. Since the object file is newer than the source AMU will not rerun CC and will exit with a zero return code.
5. Srcbuild will return with a zero return code.

So between them they hide errors, in a most unhelpful fashion. I’ll fix Srcbuild, but CC and AMU need sorting as well.