build: use the system lib luajit for s390x (#9172)

Signed-off-by: YingJie Fu <[email protected]>

README.md

@@ -104,7 +104,7 @@ Fluent Bit is fully supported on Windows environments, get started with [these i
 
 #### Running on s390x
 
-Fluent Bit is supported on Linux on IBM Z(s390x), but the WASM and LUA filter plugins are not.
+Fluent Bit runs on Linux on IBM Z(s390x), but the WASM filter plugin is not. For the LUA filter plugin, it runs when `libluajit` is installed on the system and fluent bit is built with `FLB_LUAJIT` and `FLB_PREFER_SYSTEM_LIB_LUAJIT` on.
 
 ### Plugins: Inputs, Filters and Outputs
 

cmake/s390x.cmake

@@ -3,6 +3,10 @@ if(CMAKE_SYSTEM_PROCESSOR MATCHES "^(SystemZ|s390x)")
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fsigned-char")
   message(STATUS "Cannot build WAMR on this platform, so WASM support is disabled.")
   set(FLB_WASM OFF)
-  message(STATUS "This platform does not support LuaJIT, so it's disabled.")
-  set(FLB_LUAJIT OFF)
+  if(FLB_LUAJIT)
+    if(NOT FLB_PREFER_SYSTEM_LIB_LUAJIT)
+      message(WARNING "LuaJIT is disabled, this platform does not support built-in LuaJIT. To use lua filter on this platform, install LuaJIT as system lib and build again with both FLB_LUAJIT and FLB_PREFER_SYSTEM_LIB_LUAJIT on.")
+      set(FLB_LUAJIT OFF)
+    endif()
+  endif()
 endif ()