CMake fails if `CMAKE_BUILD_TYPE` is not one of Debug, Release, MinSizeRel, and RelWithDebInfo

As the title states, CMake fails if CMAKE_BUILD_TYPE is not one of Debug, Release, MinSizeRel, and RelWithDebInfo. This would normally not be a problem, but file_options is intended to be included as a subproject using FetchContent, which means that it could make the parent project fail if it is configured with another CMAKE_BUILD_TYPE. The solution when this happens is to warn the user and change CMAKE_BUILD_TYPE to the default one.