diff --git a/cpp/CMakeLists.txt b/cpp/CMakeLists.txt
index 72b7913a6..d84633130 100644
--- a/cpp/CMakeLists.txt
+++ b/cpp/CMakeLists.txt
@@ -467,7 +467,7 @@ endif ()
 
 if (${USE_POSIX_THREAD} STREQUAL "ON" OR ((APPLE OR UNIX) AND ${USE_BOOST} STREQUAL "OFF" AND ${USE_STDMUTEX} STREQUAL "OFF"))
   if(CMAKE_USE_PTHREADS_INIT)
-    list (APPEND CMAKE_C_FLAGS "-pthread")
+    set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -pthread")
   endif()
   if(CMAKE_THREAD_LIBS_INIT)
     list (APPEND LIBRARY_DEPS ${CMAKE_THREAD_LIBS_INIT})