[cmake] Findzstd.cmake cannot find shared zstd library in win-64 conda environment when using Clang

[mgorny]

Also reported for the feedstock but I'm pretty sure the problem is in the CMake file: conda-forge/llvmdev-feedstock#306

Example Azure log from downstream package indirectly broken by this: azure-log.txt

I don't have a Windows setup to test this properly, but my reading of the code leads to the following conclusion (that I've originally stated in conda-forge/triton-feedstock#29 (comment)):

  │ │   -- Found zstd: %PREFIX%/Library/lib/libzstd.lib

https://github.com/llvm/llvm-project/blob/e21dc4bd5474d04b8e62d7331362edcc5648d7e5/llvm/cmake/modules/Findzstd.cmake

When MSVC is used, zstd_STATIC_LIBRARY_SUFFIX is _static.lib$, otherwise it's just .lib$. So when using MSVC, zstd.lib (and libzstd.lib would too) triggers the second clause in the zstd_FOUND condition, causing it to perform explicit lookup for bin/zstd.dll. On the other hand, when using Clang, it just treats libzstd.lib as a static library (the first clause) and doesn't bother beyond that. So effectively, with MSVC it finds both static and shared libraries, and with Clang only the former.

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Now, this is particularly problematic, when LLVM itself is built with MSVC (as it is in conda-forge). It uses the shared zstd library and includes references to the dependency on this library in the installed CMake files. However, when reverse dependencies use Clang instead, and try to find_package(LLVM), the lookup is performed again. Since it is now performed with Clang and it doesn't find the shared library, the dependencies are unsatisfied and CMake errors out:

 │ │   CMake Error at %BUILD_PREFIX%/Library/lib/cmake/llvm/LLVMExports.cmake:64 (set_target_properties):
 │ │     The link interface of target "LLVMSupport" contains:
 │ │       zstd::libzstd_shared
 │ │     but the target was not found.  Possible reasons include:
 │ │       * There is a typo in the target name.
 │ │       * A find_package call is missing for an IMPORTED target.
 │ │       * An ALIAS target is missing.
 │ │   Call Stack (most recent call first):
 │ │     %BUILD_PREFIX%/Library/lib/cmake/llvm/LLVMConfig.cmake:349 (include)
 │ │     %BUILD_PREFIX%/Library/lib/cmake/mlir/MLIRConfig.cmake:10 (find_package)
 │ │     CMakeLists.txt:61 (find_package)

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And perhaps the worst part, it is possible that I've actually broken that code before. I specifically see the shared library logic for MSVC has been added in e7fc754 — perhaps the condition there should check for Windows rather than MSVC. But it's outside my expertise to change that.

CC @nga888