Fix build error caused by CMAKE_CXX_FLAGS (#3204)

Co-authored-by: Haigang Xi <xhgang@blackwingasset.com>

Author: Searion

CMakeLists.txt

@@ -146,7 +146,7 @@ if(WITH_MESALINK)
 endif()
 set(CMAKE_CPP_FLAGS "${CMAKE_CPP_FLAGS} -DBTHREAD_USE_FAST_PTHREAD_MUTEX -D__const__=__unused__ -D_GNU_SOURCE -DUSE_SYMBOLIZE -DNO_TCMALLOC -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -D__STDC_CONSTANT_MACROS -DBRPC_REVISION=\\\"${BRPC_REVISION}\\\" -D__STRICT_ANSI__")
 set(CMAKE_CPP_FLAGS "${CMAKE_CPP_FLAGS} ${DEBUG_SYMBOL} ${THRIFT_CPP_FLAG}")
-set(CMAKE_CXX_FLAGS "${CMAKE_CPP_FLAGS} -O2 -pipe -Wall -W -fPIC -fstrict-aliasing -Wno-invalid-offsetof -Wno-unused-parameter -fno-omit-frame-pointer")
+set(CMAKE_CXX_FLAGS "${CMAKE_CPP_FLAGS} ${CMAKE_CXX_FLAGS} -O2 -pipe -Wall -W -fPIC -fstrict-aliasing -Wno-invalid-offsetof -Wno-unused-parameter -fno-omit-frame-pointer")
 set(CMAKE_C_FLAGS "${CMAKE_CPP_FLAGS} -O2 -pipe -Wall -W -fPIC -fstrict-aliasing -Wno-unused-parameter -fno-omit-frame-pointer")
 
 macro(use_cxx11)