SheepShaverLauncher: Use default compiler settings

The previous Mac SDK setting, 10.4, is very old and does not allow compilation
on recent versions of OS X. Same with the compiler, gcc 4.0.

Since SheepShaver doesn't purposefully target a specific SDK or compiler,
it's better to just use XCode's default, which should always be usable.
If a user wants a specific SDK, they can set it themselves.
SheepShaverLauncher: Compile on 10.6+ SDK

On Mac OS X 10.6, many informal protocols have become formal, and failure to
declare conformance to them is an error. Declare our delegates and data sources
to be conformant, conditional on an SDK >= 10.6