diff --git a/cmake/modules/AddLLVM.cmake b/cmake/modules/AddLLVM.cmake
index 96246fafe4a..0236746b28a 100755
--- a/cmake/modules/AddLLVM.cmake
+++ b/cmake/modules/AddLLVM.cmake
@@ -181,17 +181,19 @@ function(add_unittest test_suite test_name)
 
   include_directories(${LLVM_MAIN_SRC_DIR}/utils/unittest/googletest/include)
   set_property(TARGET ${test_name} APPEND PROPERTY COMPILE_DEFINITIONS GTEST_HAS_RTTI=0)
-  if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
-    set_property(TARGET ${test_name} APPEND_STRING PROPERTY COMPILE_FLAGS " -fno-rtti")
-  elseif (MSVC)
-    set_property(TARGET ${test_name} APPEND_STRING PROPERTY COMPILE_FLAGS " /GR-")
-  endif ()
-
   if (NOT LLVM_ENABLE_THREADS)
     set_property(TARGET ${test_name} APPEND PROPERTY COMPILE_DEFINITIONS GTEST_HAS_PTHREAD=0)
   endif ()
 
-  if (SUPPORTS_NO_VARIADIC_MACROS_FLAG)
-    set_property(TARGET ${test_name} APPEND_STRING PROPERTY COMPILE_FLAGS " -Wno-variadic-macros")
+  get_property(target_compile_flags TARGET ${test_name} PROPERTY COMPILE_FLAGS)
+  if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
+    set(target_compile_flags "${target_compile_flags} -fno-rtti")
+  elseif (MSVC)
+    set(target_compile_flags "${target_compile_flags} /GR-")
   endif ()
+
+  if (SUPPORTS_NO_VARIADIC_MACROS_FLAG)
+    set(target_compile_flags "${target_compile_flags} -Wno-variadic-macros")
+  endif ()
+  set_property(TARGET ${test_name} PROPERTY COMPILE_FLAGS "${target_compile_flags}")
 endfunction()