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

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.

src/Compilation.zig

@@ -5734,6 +5734,10 @@ pub fn addCCArgs(
                 },
             }
 
+            if (mod.optimize_mode != .Debug) {
+                try argv.append("-Werror=date-time");
+            }
+
             if (target_util.supports_fpic(target) and mod.pic) {
                 try argv.append("-fPIC");
             }