build: add a workaround for CMake <3.16 on Windows
build: add a workaround for CMake <3.16 on Windows

The import library is being emitted into the wrong location on CMake
3.15.1.  Add a workaround to enable use of LLBuild for building s-p-m on
Windows.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild