build: Address fallbacking issue of finding libzstd

Signed-off-by: Hiroshi Hatake <hiroshi@chronosphere.io>

Author: cosmo0920

CMakeLists.txt

@@ -602,8 +602,28 @@ if(FLB_PREFER_SYSTEM_LIB_ZSTD)
   find_package(ZSTD 1.4.8 QUIET)
   if(ZSTD_FOUND)
     set(LIBZSTD_FOUND TRUE)
-    set(LIBZSTD_LIBRARIES BUNDLED::ZSTD)
-  else()
+    # Use the target provided by find_package(ZSTD)
+    # Modern FindZSTD provides ZSTD::libzstd_static or ZSTD::libzstd_shared
+    if(TARGET ZSTD::libzstd_static)
+      set(LIBZSTD_LIBRARIES ZSTD::libzstd_static)
+    elseif(TARGET ZSTD::libzstd_shared)
+      set(LIBZSTD_LIBRARIES ZSTD::libzstd_shared)
+    elseif(TARGET ZSTD::ZSTD)
+      set(LIBZSTD_LIBRARIES ZSTD::ZSTD)
+    else()
+      # Fallback: create a wrapper if no target is provided
+      add_library(BUNDLED::ZSTD INTERFACE IMPORTED)
+      if(ZSTD_LIBRARIES)
+        set_property(TARGET BUNDLED::ZSTD PROPERTY
+          INTERFACE_LINK_LIBRARIES "${ZSTD_LIBRARIES}")
+      endif()
+      if(ZSTD_INCLUDE_DIRS)
+        set_property(TARGET BUNDLED::ZSTD PROPERTY
+          INTERFACE_INCLUDE_DIRECTORIES "${ZSTD_INCLUDE_DIRS}")
+      endif()
+      set(LIBZSTD_LIBRARIES BUNDLED::ZSTD)
+    endif()
+   else()
     find_package(PkgConfig)
     pkg_check_modules(LIBZSTD libzstd>=1.4.8)
     if(LIBZSTD_FOUND)