Fix `CMAKE_BUILD_TYPE` handling in CMakeLists.txt in the root directory

[ahuimmo]

In CMakeLists.txt in the root directory, at line 41:

if(NOT DEFINED ${CMAKE_BUILD_TYPE})
    set(CMAKE_BUILD_TYPE "Release")
endif()

If I want a debug build, and I run:

$ cmake .. -DCMAKE_BUILD_TYPE=Debug

Since ${CMAKE_BUILD_TYPE} is "Debug", and the code is actually checking if a variable named "Debug" is defined, and obviously, it isn't. So I always get a release build instead. I think it should be:

if(NOT DEFINED CMAKE_BUILD_TYPE)

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