This is because target_link_options was added in v3.13
So this wouldn't build with cmake v3.12
Likewise in CMAKE_CXX_STANDARD only supports value of 17 starting with
cmake v3.9
So the test wouldn't build properly with cmake v3.6
The CMake buildsystem should be completely reviewed to properly match
what is done by SCons, instead of making its own arbitrary decisions on
how godot-cpp should be compiled.
Currently the SCons setup doesn't include warning options, so CMake
shouldn't either. Options similar to upstream Godot's SCons setup could
be added, and then replicated for CMake.
Changes the `<godot/gdextension_interface.h>` include to simply
`<gdextension_interface.h>`.
Refactor and better document the SCons and CMake logic around setting
the paths to the header and API JSON file.
