[CMake] Followup for r337366: Only export LLVM_LINK_LLVM_DYLIB if it's set to ON

Summary:
As it was, always exporting LLVM_LINK_LLVM_DYLIB caused out-of-tree
clients to lose the ability to link against the dylib, even if in-tree tools did
not. By only exporting the setting if it is enabled, out-of-tree clients get the
correct default, but may still choose if they can.

Reviewers: mgorny, beanz, labath, bogner, chandlerc

Reviewed By: bogner, chandlerc

Subscribers: bollu, llvm-commits

Differential Revision: https://reviews.llvm.org/D49843

llvm-svn: 338119