[SoM] SoM should have an option for the OS configuration to target or intuit it

[stephen-hawley]

Right now we don't do anything special about the target OS and we don't have options for that.
The class CustomSwiftCompiler handles compiling the wrappers.

We should have code that can intuit the target OS version from the input and tell the compiler that's what we want and we should probably have an option that can override that on the command line.

The code that does the actual compilation is in CustomSwiftCompiler and the code that feeds it is in WrappingCompiler