Error when building linux ssd demo

[zhaofenqiang]

I followed this guide to build the ssd demo on Ubuntu 16.04.
Everything is ok untill make , the error is:

/home/zfq/Mobile/Demo/linux/paddle_image_recognizer.h:23:59: error: ‘const char* paddle_error_string(paddle_error)’ was declared ‘extern’ and later ‘static’ [-fpermissive]

I am new at C++, so could anyone help me, please, thanks!