A mistake that seems to be quite common, which I currently experienced whilst investigating a `DBUG_SYNC` test failure reported by monty, is that it is actually possible to pass ` -DCMAKE_BUILD_TYPE=mysql_release` to cmake, even though `mysql_release` is a `BUILD_CONFIG` not a `BUILD_TYPE`.
In this case the build continues just fine, but all sorts of undefined things happen with the server code because odd combinations of `#define` happen when they shouldn't.

We should make cmake fail if an unsupported option is passed as `BUILD_TYPE`.