{CMake} Multi-config builds do not work

[asmaloney]

CMAKE_BUILD_TYPE applies to single-config builds, but not multi-config (e.g. Visual Studio, XCode) ones. For those we should be using generator expressions.

The fix for this is not obvious at the moment because of the way the template files are processed and because godot-cpp itself does not work properly for multi-config.

I have submitted fixes for godot-cpp in this PR.

Once it's fixed in godot-cpp I will revisit how the templates work here.