Review comment

Author: HyukjinKwon

cpp/cmake_modules/SetupCxxFlags.cmake

@@ -496,7 +496,7 @@ if(ARROW_CPU_FLAG STREQUAL "x86")
       message(FATAL_ERROR "AVX2 required but compiler doesn't support it.")
     endif()
     list(JOIN ARROW_AVX2_FLAGS " " ARROW_AVX2_FLAGS_COMMAND_LINE)
-    set(CXX_COMMON_FLAGS "${CXX_COMMON_FLAGS} ${ARROW_AVX2_FLAGS_COMMAND_LINE}")
+    string(APPEND CXX_COMMON_FLAGS " ${ARROW_AVX2_FLAGS_COMMAND_LINE}")
     add_definitions(-DARROW_HAVE_AVX2 -DARROW_HAVE_BMI2 -DARROW_HAVE_SSE4_2)
   elseif(ARROW_SIMD_LEVEL STREQUAL "SSE4_2")
     if(NOT CXX_SUPPORTS_SSE4_2)