Always enforce optimization when building for MSVC

In a couple of places there's code like if(CONFIG_something) { ... }. On MSVC, with optimizations disabled, the supposedly-disabled blocks always generate code, including references to functions which are not available, causing link errors. In other words, some level of optimization is always required to correctly build things - so enforce optimization in all cases.

Would there be another way, should disabling optimization be really needed? Yes - replace if(CONFIG_something) { ... } with the matching preprocessor check (#if CONFIG_something ... #endif). However, this would require source code to be touched. But I'm assuming deviating from upstream is undesirable, hence the attempt to solve it via build flags.