Added -pthread to CFLAGS for linux

examples/big_upload/Makefile

examples/cookie_authentication/Makefile

examples/digest_authentication/Makefile

examples/file_upload/Makefile

examples/form_submit/Makefile

examples/hello_world/Makefile

examples/http_client/Makefile

examples/mjpg_streamer/Makefile

examples/multi_threaded_server/Makefile

examples/proxy_server/Makefile

examples/restful_api/Makefile

examples/send_file/Makefile

examples/web_server/Makefile

examples/websocket_chat/Makefile

examples/websocket_echo_server/Makefile

examples/websocket_ssl_proxy/Makefile