[llvm] fix zstd_shared detection on Windows (PR #139945)

[Martin Storsjö via llvm-commits]

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@@ -29,11 +29,11 @@ find_package_handle_standard_args(
 )
 
 if(zstd_FOUND)
-  if(zstd_LIBRARY MATCHES "${zstd_STATIC_LIBRARY_SUFFIX}$")
+  if(zstd_LIBRARY MATCHES "${zstd_STATIC_LIBRARY_SUFFIX}$" AND NOT zstd_LIBRARY MATCHES "\\.dll\\.a$")
     set(zstd_STATIC_LIBRARY "${zstd_LIBRARY}")
   elseif (NOT TARGET zstd::libzstd_shared)
     add_library(zstd::libzstd_shared SHARED IMPORTED)
-    if(MSVC OR "${CMAKE_CXX_SIMULATE_ID}" STREQUAL "MSVC")
+    if(WIN32 OR CYGWIN)
       include(GNUInstallDirs) # For CMAKE_INSTALL_LIBDIR and friends.
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mstorsjo wrote:

Can you briefly explain what the code within this conditional branch does? (It's quite unreadable cmake soup as is...) I presume it does some transformation which is essential for using zstd in a mingw build setting.

Is it essential only for using the output of llvm-config, or for building llvm itself as well?

https://github.com/llvm/llvm-project/pull/139945