[CMake] Only export the LLVM_LINK_LLVM_DYLIB setting if not yet set

[guitargeek]

Follows up on these two commits:

• 3b8b3c2 [CMake] Export the LLVM_LINK_LLVM_DYLIB setting
• 5fed24a [CMake] Followup for r337366: Only export LLVM_LINK_LLVM_DYLIB if it's set to ON

Commit 3b8b3c2 introduced the export of the LLVM_LINK_LLVM_DYLIB setting, so that clients can check whether the option of dynamic linking is available. However, it left no way for the client to actually set LLVM_LINK_LLVM_DYLIB themselves.

Commit 5fed24a addressed the problem that out-of-tree clients lost the ability to link against the dylib, even if in-tree tools did not.

There is one remaining problem: if LLVM_LINK_LLVM_DYLIB is supported there is no way to configure clients to not link against llvm dynamically anymore. Naively, one would be able to do this for Clang e.g. with -DLLVM_LINK_LLVM_DYLIB=OFF.

This commit suggests to fix this problem by only setting the DYLIB option if it was not already set before.

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[hahnjo]

Note that downstream users can choose by specifying DISABLE_LLVM_LINK_LLVM_DYLIB in their CMake

[guitargeek]

Yes that's true, but if you want to make that configurable, the downstream package (in this case e. g. standalone Clang) would need another flag so you can steer that behavior when building the downstream package. Of course that's also an option, but I think making this change in LLVM is more sustainable because Clang is not the only downstream package affected by this

[guitargeek]

Note that downstream users can choose by specifying DISABLE_LLVM_LINK_LLVM_DYLIB in their CMake

Maybe to clarify possible confusion from my previous comment: the DISABLE_LLVM_LINK_LLVM_DYLIB argument is not a configuration option for building downstream packages, but a flag that you can set in llvm_add_library: https://github.com/llvm/llvm-project/blob/main/llvm/cmake/modules/AddLLVM.cmake#L500

So if you want to build e.g. Clang with disabled dynamic linking, doing -DDISABLE_LLVM_LINK_LLVM_DYLIB as you imply would not help because this argument is hardcoded in the llvm_add_library calls.