diff --git a/CMakeLists.txt b/CMakeLists.txt
index ab41f5fa..c36c873d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -51,7 +51,7 @@ check_c_source_compiles("
 find_package(Threads)
 if(Threads_FOUND)
   if(CMAKE_USE_PTHREADS_INIT)
-    list(APPEND CMAKE_C_FLAGS "-pthread")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -pthread")
   endif()
   foreach(PTHREAD_TEST HAVE_PTHREAD_PRIO_INHERIT PTHREAD_CREATE_UNDETACHED)
     check_c_source_compiles("
@@ -177,7 +177,7 @@ foreach(I_SIMD RANGE ${WEBP_SIMD_FLAGS_RANGE})
       include(CheckCCompilerFlag)
       check_c_compiler_flag(${SIMD_COMPILE_FLAG} HAS_COMPILE_FLAG)
       if(HAS_COMPILE_FLAG)
-        list(APPEND CMAKE_C_FLAGS ${SIMD_COMPILE_FLAG})
+        set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${SIMD_COMPILE_FLAG}")
       endif()
     endif()
   endif()