CMake package does not provide version

Specifying the version in `find_package` results in an error message saying that the version is unknown.

```cmake
find_package(glbinding 3.5.0 REQUIRED)
```

```
Could not find a configuration file for package "glbinding" that is
compatible with requested version "3.5.0".

The following configuration files were considered but not accepted:

/usr/local/share/glbinding/glbinding-config.cmake, version: unknown
```

I installed glbinding through the following steps
```
git clone https://github.com/cginternals/glbinding
cd glbinding
cmake -S . -B build
cmake --build ./build
cmake --install ./build
```

Reproduce the problem using the reported git clone, cmake -S . -B build, build, and install steps, then inspect the generated /usr/local/share/glbinding/glbinding-config.cmake and the CMake packaging configuration that produces it. Done means find_package(glbinding 3.5.0 REQUIRED) accepts the installed package and reports its actual version instead of “unknown”.

Written by the indexing model from the issue text.