diff --git a/libcxx/cmake/config-ix.cmake b/libcxx/cmake/config-ix.cmake
--- a/libcxx/cmake/config-ix.cmake
+++ b/libcxx/cmake/config-ix.cmake
@@ -38,7 +38,7 @@
     list(APPEND CMAKE_REQUIRED_LIBRARIES c)
   endif ()
   if (LIBCXX_USE_COMPILER_RT)
-    list(APPEND CMAKE_REQUIRED_FLAGS -rtlib=compiler-rt)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -rtlib=compiler-rt")
     find_compiler_rt_library(builtins LIBCXX_BUILTINS_LIBRARY)
     list(APPEND CMAKE_REQUIRED_LIBRARIES "${LIBCXX_BUILTINS_LIBRARY}")
   elseif (LIBCXX_HAS_GCC_LIB)
diff --git a/libcxxabi/cmake/config-ix.cmake b/libcxxabi/cmake/config-ix.cmake
--- a/libcxxabi/cmake/config-ix.cmake
+++ b/libcxxabi/cmake/config-ix.cmake
@@ -28,7 +28,7 @@
     list(APPEND CMAKE_REQUIRED_LIBRARIES c)
   endif ()
   if (LIBCXXABI_USE_COMPILER_RT)
-    list(APPEND CMAKE_REQUIRED_FLAGS -rtlib=compiler-rt)
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -rtlib=compiler-rt")
     find_compiler_rt_library(builtins LIBCXXABI_BUILTINS_LIBRARY)
     list(APPEND CMAKE_REQUIRED_LIBRARIES "${LIBCXXABI_BUILTINS_LIBRARY}")
   else ()