[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