Add configuration for silent build

It would be nice to have a possibility to suppress the cmake output.

Right now, the cmake output is forwarded to cargo. If the output contains non-utf-8 characters, the build fails. This can happen when using e.g. a german msvc that outputs german umlauts in comments.
I have filed an issue for cargo as well, asking them to ignore those lines.

[alexcrichton]

Sounds reasonable to me! This should definitely be fixed in Cargo in any case as well.