Unbreak building on Linux without sendfile64() support. It was broken in 8e446a2daf48 when the NGX_SENDFILE_LIMIT constant was added to ngx_linux_sendfile_chain.c having the same name as already defined one in ngx_linux_config.h. The newer is needed to overcome a bug in old Linux kernels by limiting the number of bytes to send per sendfile() syscall. The older is used with sendfile() on ancient kernels that works with 32-bit offsets only. One of these renamed to NGX_SENDFILE_MAXSIZE.