I have csharp-sqlite both building and running successfully with Mono on Mac OS X, without breaking the Windows build (i.e., I can compile on Windows and use on Mac OS X, and visa versa).
I need to think a bit more about db file locking on Linux/Unix and Mac OS X, but for now it is good if you don't have 2 processes writing to the DB simultaneously on those platforms.
I *could* supply settings to hide the build warnings - but it is probably better to fix the causes instead. Many of the warnings are for genuinely unused variables and are trivial to fix. Many warnings report unreachable code and most of these can be fixed by tweaking the #if/#endif constructs. Locally, I've made a start on both of those clean-up operations.
I've requested permission from my employer to push these patches upstream, and am just waiting for an answer now.
Stewart.