Avoid overwriting CMAKE_CXX_FLAGS

Currently, we can't specify new `CMAKE_CXX_FLAGS` from the command line
of invoking cmake due to we overwrote the flags in CMakeLists.txt.

This patch avoids this.

CMakeLists.txt

@@ -139,7 +139,7 @@ endif()
 # End Detecing Uthread
 
 
-string(REPLACE ";" " " CMAKE_CXX_FLAGS "${CXX_FLAGS}")
+string(REPLACE ";" " " CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${CXX_FLAGS}")
 
 if(NOT CMAKE_CXX_COMPILER_ID MATCHES "MSVC")
     set(CMAKE_CXX_FLAGS_DEBUG "-O0")