The Unity runtime for Google Native Client uses the Mono library, licensed 
under the LPGL v2. 

The source code used for building mono for native client can be found at 
https://github.com/elijahtaylor/mono/tree/1ca063d2a5a13adf7ec88711a1d01e5cd1dd8106

Build scripts and instructions for building for native client can be found in 
the "nacl" directory in that repository.