diff --git a/cmake/FindLibClang.cmake b/cmake/FindLibClang.cmake
index 87c0311..5ae891c 100644
--- a/cmake/FindLibClang.cmake
+++ b/cmake/FindLibClang.cmake
@@ -91,7 +91,7 @@ else()
 endif()
 
 if(CLANG_COMPILER AND NOT LIBCLANG_CXX_FLAGS)
-  set(LIBCLANG_FLAGS_DETECTION_COMMAND "${CLANG_COMPILER} $ENV{CXXFLAGS} -E -x c++ ${CLANG_OPT} -v -")
+  set(LIBCLANG_FLAGS_DETECTION_COMMAND "${CLANG_COMPILER} ${CMAKE_CXX_FLAGS} -E -x c++ ${CLANG_OPT} -v -")
   separate_arguments(LIBCLANG_FLAGS_DETECTION_COMMAND)
   execute_process(COMMAND ${LIBCLANG_FLAGS_DETECTION_COMMAND}
                   INPUT_FILE /dev/null