autofs: allow to use libtirpc

autofs: allow to use libtirpc

uClibc-ng removed internal RPC implementation as it
is ipv4 only and can not be used for most important RPC software
rpcbind and nfs-utils.
musl does not implement RPC and GNU C library deprecated the
internal implementation a while ago.
It is still possible to use the C library implementation.

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
[Thomas: change to use libtirpc when available, not just when the
toolchain does not have RPC support.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>