[CMake] Enable autolinking when compiling llbuildSwift
[CMake] Enable autolinking when compiling llbuildSwift

This will let clients avoid passing -l flag when they import
llbuildSwift.

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