There are situations where you want to build against a libc that is out
of tree or that is not the system libc (Or for cross build scenarios).
This is a change for passing the -sdk and include paths for things like
this.
The pthread dependency has been lifted through the Windows port
generalising the SwiftThreadExtras. Enable building these unit test
binaries which are needed for the validation test suite.
The key thing here is that all of the underlying code is exactly the same. I
purposely did not debride anything. This is to ensure that I am not touching too
much and increasing the probability of weird errors from occurring. Thus the
exact same code should be executed... just the routing changed.
76ef276 made the compiler autolink libraries more eagerly than before.
This normally wouldn't affect the products we build with CMake, but
when an overlay is autolinked the compiler also puts in a dummy symbol
to make sure it gets linked. So now we have to link to the overlay
directly.
From the Swift documentation:
"If you define an optional variable without providing a default value,
the variable is automatically set to nil for you."
