Compiler warnings treated as errors

[ceeac]

If the SIMUTRANS_WARNINGS_AS_ERRORS CMake option is enabled, compiler warnings are treated as errors to prevent easily-fixable bugs from creeping in. The option is disabled by default, but since r12205 (from today) enabling this will no longer break the build for GCC and Clang. Therefore, I want to enable this option for the relevant nightly (CI) build jobs at some point in the future (probably next week or so). For non-nightly builds the option will still be disabled to not accidentally break the build for people who are self-compiling Simutrans.

[victor_18993]

I agree. Enabling this for the relevant CI/nightly builds now that GCC and Clang are clean sounds like a good step.