[cmake] Fix Findzstd.cmake to handle OpenBSD shared libraries

Fix Findzstd CMake to handle shared libraries on OpenBSD correctly.
This userland does not use shared library symlinks without SOVERSION,
so the result of find_library() does not ever end with
zstd_SHARED_LIBRARY_SUFFIX.  To work around this, reverse the logic
to compare the result against zstd_STATIC_LIBRARY_SUFFIX and assume
shared library otherwise.

While at it, fix the conditions not to fall back to "result is static
library" path if it actually was recognized as a shared library but
zstd_shared target already existed.

Fixes #59056

Differential Revision: https://reviews.llvm.org/D138361

Author: mgorny

llvm/cmake/modules/Findzstd.cmake

@@ -11,10 +11,8 @@
 # zstd::libzstd_static
 
 if(MSVC)
-  set(zstd_SHARED_LIBRARY_SUFFIX "\\${CMAKE_LINK_LIBRARY_SUFFIX}$")
   set(zstd_STATIC_LIBRARY_SUFFIX "_static\\${CMAKE_STATIC_LIBRARY_SUFFIX}$")
 else()
-  set(zstd_SHARED_LIBRARY_SUFFIX "\\${CMAKE_SHARED_LIBRARY_SUFFIX}$")
   set(zstd_STATIC_LIBRARY_SUFFIX "\\${CMAKE_STATIC_LIBRARY_SUFFIX}$")
 endif()
 
@@ -31,8 +29,9 @@ find_package_handle_standard_args(
 )
 
 if(zstd_FOUND)
-  if(zstd_LIBRARY MATCHES "${zstd_SHARED_LIBRARY_SUFFIX}$" AND
-     NOT TARGET zstd::libzstd_shared)
+  if(zstd_LIBRARY MATCHES "${zstd_STATIC_LIBRARY_SUFFIX}$")
+    set(zstd_STATIC_LIBRARY "${zstd_LIBRARY}")
+  elseif (NOT TARGET zstd::libzstd_shared)
     add_library(zstd::libzstd_shared SHARED IMPORTED)
     if(MSVC)
       # IMPORTED_LOCATION is the path to the DLL and IMPORTED_IMPLIB is the "library".
@@ -51,8 +50,6 @@ if(zstd_FOUND)
           INTERFACE_INCLUDE_DIRECTORIES "${zstd_INCLUDE_DIR}"
           IMPORTED_LOCATION "${zstd_LIBRARY}")
     endif()
-  else()
-    set(zstd_STATIC_LIBRARY "${zstd_LIBRARY}")
   endif()
   if(zstd_STATIC_LIBRARY MATCHES "${zstd_STATIC_LIBRARY_SUFFIX}$" AND
      NOT TARGET zstd::libzstd_static)
@@ -63,7 +60,6 @@ if(zstd_FOUND)
   endif()
 endif()
 
-unset(zstd_SHARED_LIBRARY_SUFFIX)
 unset(zstd_STATIC_LIBRARY_SUFFIX)
 
 mark_as_advanced(zstd_INCLUDE_DIR zstd_LIBRARY zstd_STATIC_LIBRARY)