Use this to define the macro location rather than the "host" dir (which
is actually for the build and not the host). Furthermore, on Windows,
the build dir is /usr/lib/swift as the host content is in the SDK.
This prepares the tests for Windows.
The macro tests were all using "REQUIRES: OS=macosx" as a proxy for
"have the Swift Swift parser". There was an existing feature for this,
but it was just checking whether the path was passed through. Fix that
to use the same variable as in CMake.
Also remove all extraneous `-I` and `-L` to the host libs in the target
invocations.
