srs demo requires ffmpeg to transcode.

@@ -4,8 +4,9 @@ if [[ ! -d $src_dir ]]; then echo "错误：必须在src同目录执行脚本";
 
 # step 1: build srs 
 # nginx is not needed, we use cherrypy to delivery hls.
+# demo needs ffmpeg to transcode the live stream.
 echo "编译SRS"
-./configure --x86-x64 && make
+./configure --x86-x64 --with-ffmpeg && make
 ret=$?; if [[ 0 -ne $ret ]]; then echo "错误：编译SRS失败"; exit $ret; fi
 
 echo "编译SRS成功"