C sharp missing library finding support

C# is missing library finding support. For example when looking for `Mono.Posix` or `Mono.CSharp` libraries in the following way doesn't work:

```
csc = meson.get_compiler('cs')
csc.find_library('Mono.Posix')
```

This gives the following warning: `Language C sharp does not support library finding.`

Start at the `meson.get_compiler('cs')` entry point and trace how `csc.find_library('Mono.Posix')` is handled. Compare the C# path with library-finding support in other compiler languages; done means the example no longer emits the unsupported-library warning and can find the requested libraries.

Written by the indexing model from the issue text.