compiler: Enable -Werror=date-time for C/C++ code in release builds

[alexrp]

We advertise reproducible builds for release modes, so let's help users achieve that in C/C++ code. Users can still override this manually if they really want.

[alexrp]

I could also make this -Werror=date-time, assuming we like this change. Done.

[andrewrk]

Nice idea. Here's my counter-proposal.

The langref says that Debug is not required to make reproducible builds, but all the release modes are required to make reproducible builds. There is currently no option for changing this with compilation flags. Let's start by continuing to not having an option to override this, and do what zig cc always does: try to give you the zig build mode even when compiling C source files.

In this case, it means that we do, in fact, set -Werror=date-time for release builds, because that is required for reproducible builds. Users who want to opt out of this can do so by directly overriding this flag because we put the user-specified clang flags after the ones generated by Zig.

[alexrp]

Ok, changed it to simply always pass -Werror=date-time if the module's mode is not Debug.

[andrewrk]

Thanks. I think that commit message would have also worked well as a comment, but it's good enough that someone can use git blame to find it.