I have installed pdf-tools by melpha, and write a line in init.el file to load it. However, when I opened my pdf files I still couldn't view it and the error of "Error running timer ‘pdf-cache--prefetch-start’: (error "PNG image supported not compiled into Emacs")". It seems that I didn't have PNG support. When I hit C-c C-c, it shifted into DocView mode. Could anyone give me some suggestions to solve this?