diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8133c22ac5b..0733a823320 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -236,7 +236,7 @@ endif()
 target_link_libraries(supertux2 supertux2_lib)
 
 set_target_properties(supertux2_lib PROPERTIES OUTPUT_NAME supertux2_lib)
-set_target_properties(supertux2_lib PROPERTIES COMPILE_FLAGS "${SUPERTUX2_EXTRA_WARNING_FLAGS}")
+set_source_files_properties(${SUPERTUX_SOURCES_CXX} PROPERTIES COMPILE_OPTIONS "${SUPERTUX2_EXTRA_WARNING_FLAGS}")
 if(EMSCRIPTEN)
   target_link_options(supertux2 PUBLIC -sEXPORTED_FUNCTIONS=['_main','_set_resolution','_save_config','_onDownloadProgress','_onDownloadFinished','_onDownloadError','_onDownloadAborted','_getExceptionMessage'] PUBLIC -sEXPORTED_RUNTIME_METHODS=['ccall','cwrap'])
 endif()
diff --git a/mk/cmake/SuperTux/WarningFlags.cmake b/mk/cmake/SuperTux/WarningFlags.cmake
index 1df05966564..36847e160c3 100644
--- a/mk/cmake/SuperTux/WarningFlags.cmake
+++ b/mk/cmake/SuperTux/WarningFlags.cmake
@@ -9,7 +9,7 @@ if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
   set(CMAKE_LD_FLAGS_PROFILE "-lgmon" CACHE STRING "Profile flags")
 
   if(WERROR)
-    string(CONCAT SUPERTUX2_EXTRA_WARNING_FLAGS "-Werror ")
+    string(CONCAT SUPERTUX2_EXTRA_WARNING_FLAGS "-Werror;")
   endif()
 
   if(WARNINGS)
@@ -75,7 +75,7 @@ if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
     foreach(flag IN LISTS ALL_WARNING_FLAGS)
       check_cxx_compiler_flag(${flag} HAVE_FLAG_${flag})
       if(HAVE_FLAG_${flag})
-        string(APPEND SUPERTUX2_EXTRA_WARNING_FLAGS "${flag} ")
+        string(APPEND SUPERTUX2_EXTRA_WARNING_FLAGS "${flag};")
       endif()
       unset(HAVE_FLAG CACHE)
     endforeach(flag IN LISTS ALL_WARNING_FLAGS)
@@ -87,9 +87,7 @@ if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
         check_cxx_compiler_flag(-Wpoison-system-directories HAVE_POISON_SYSTEM_DIRECTORIES_WARNING)
     endif()
     if(HAVE_POISON_SYSTEM_DIRECTORIES_WARNING)
-      string(CONCAT SUPERTUX2_EXTRA_WARNING_FLAGS
-        "${SUPERTUX2_EXTRA_WARNING_FLAGS} "
-        "-Wno-poison-system-directories ")
+      string(APPEND SUPERTUX2_EXTRA_WARNING_FLAGS "-Wno-poison-system-directories;")
     endif()
    endif()
 endif()