Put contrib/libs/glslang under yamaker once again

And mark `GPL-3.0-only WITH Bison-exception-2.2` as it reads as follows:

```
As a special exception, you may create a larger work that contains
part or all of the Bison parser skeleton and distribute that work
under terms of your choice, so long as that work isn't itself a
parser generator using the skeleton or a modified version thereof
as a parser skeleton.
```

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