[cmake] On Win, do not always-append to cling-compiledata.h.in:
[cmake] On Win, do not always-append to cling-compiledata.h.in:

Before, cling-compiledata.h caused
```
interpreter\cling\lib\Interpreter\cling-compiledata.h(6,1): warning C4005: 'CLING_UCRT_VERSION': macro redefinition
  interpreter\cling\lib\Interpreter\cling-compiledata.h(3): message : see previous definition of 'CLING_UCRT_VERSION'
```
due to it containing
```

      #define CLING_INCLUDE_PATHS ""
      #define CLING_UCRT_VERSION  ""
      #define CLING_INCLUDE_PATHS ""
      #define CLING_UCRT_VERSION  "10.0.19041.0"
```

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