Use UndefinedBehaviorSanitizer when the UBSan configure checks pass, rather than the ASan configure checks
configure: stop using UBSan when the compiler only supports ASan
When activating the undefined behaviour sanitiser (UBSan), configure.ac checked the address sanitiser (ASan) variables, instead of the UBSan variables.
Fixes bug (this one); bugfix on 0.2.9.1-alpha.