CMake >3.20 warns on missing policy

```
CMake Warning (dev) at ext/glbinding/CMakeLists.txt:47 (include):
Policy CMP0120 is not set: The WriteCompilerDetectionHeader module is
removed. Run "cmake --help-policy CMP0120" for policy details. Use the
cmake_policy command to set the policy and suppress this warning.

This warning is for project developers. Use -Wno-dev to suppress it.
```

Calling `set_policy(CMP0120 NEW)` in the root CMakelists.txt results in:
```
CMake Error at ext/glbinding/source/glbinding-aux/CMakeLists.txt:179 (file):
file COPY cannot find
"A://ext/glbinding/source/codegeneration/glbinding-aux_features.h":
No such file or directory.
```