Avoid warnings in mac builds

[utelle]

I adjusted bld/cb.cs in branch sqlite3mc of my fork of your cb repo, so that the mac builds no longer produce rather large numbers of warnings. This allows to detect "real" compile issues more easily.

Because I removed also almost all code that is not necessary for building the sqlite3mc libraries (to reduce usage of computing resources), you can't take the cb.cs file as is, but I can prepare a PR, if you want me to.