set_property could not find CACHE variable CMAKE_BUILD_TYPE.

this happens when adding as a external project in a multi-config generator

the following fixes this
```CMake
if(NOT CMAKE_BUILD_TYPE)
    set(CMAKE_BUILD_TYPE Release CACHE STRING "" FORCE)
    message(STATUS "CMAKE_BUILD_TYPE is not specified, default to Release. Note that this is only relevant for single-configuration generators (such as Makefile Generators and Ninja).")
endif()
```