build: Link system libzstd when enabling in_systemd plugin

This is because RHEL10 or other distributions which use zstd compression
inside of systemd, the symbols could be collided and we need to unify
the using zstd symbols.
This is just for changing the way to linking libzstd libraries.
No need to change for building rules in packaging.

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

Author: cosmo0920

CMakeLists.txt

@@ -577,18 +577,6 @@ add_subdirectory(${FLB_PATH_LIB_MPACK} EXCLUDE_FROM_ALL)
 # Miniz (zip)
 add_subdirectory(${FLB_PATH_LIB_MINIZ} EXCLUDE_FROM_ALL)
 
-# Zstd (zstd)
-if(FLB_PREFER_SYSTEM_LIB_ZSTD)
-  find_package(PkgConfig)
-  pkg_check_modules(LIBZSTD libzstd>=1.4.8)
-endif()
-if(LIBZSTD_FOUND)
-  include_directories(${LIBZSTD_INCLUDE_DIRS})
-  link_directories(${LIBZSTD_LIBRARY_DIRS})
-else()
-  include(cmake/zstd.cmake)
-endif()
-
 # ring buffer library
 add_subdirectory(${FLB_PATH_LIB_RING_BUFFER} EXCLUDE_FROM_ALL)
 
@@ -886,6 +874,22 @@ else()
   FLB_OPTION(FLB_IN_SYSTEMD OFF)
 endif()
 
+# Zstd (zstd)
+if(FLB_PREFER_SYSTEM_LIB_ZSTD OR (FLB_SYSTEM_LINUX AND FLB_IN_SYSTEMD))
+  find_package(PkgConfig)
+  pkg_check_modules(LIBZSTD libzstd>=1.4.8 QUIET)
+  if (NOT LIBZSTD_FOUND)
+    pkg_check_modules(LIBZSTD zstd>=1.4.8 QUIET)
+  endif()
+endif()
+if(LIBZSTD_FOUND)
+  message(STATUS "Using system libzstd: ${LIBZSTD_VERSION}")
+  include_directories(${LIBZSTD_INCLUDE_DIRS})
+  link_directories(${LIBZSTD_LIBRARY_DIRS})
+else()
+  include(cmake/zstd.cmake)
+endif()
+
 # Valgrind support
 check_c_source_compiles("
   #include <valgrind/valgrind.h>