Change -Wundef warning to an error
This should catch undefined preprocessor expressions, i.e. instead of showing a warning if FOO_BAR_H is undefined: #if FOO_BAR_H
make the build fai lwith an error instead.
This combined with using #cmakedefine01 (instead of #cmakedefine), could catch things like: pim/kdepim-runtime@05a0e5bb