[LinuxCNC/linuxcnc PR#587] Fallback to building with TI-RPC if RPC is not provided by libc

This commit updates autoconf to attemt building with TI-RPC if libc does
not provide RPC.

The RPC implementation in libc was obsoleted in 2011. Ref:

https://sourceware.org/git/?p=glibc.git;a=commit;h=7b57bfe5988e476ea40934457dfd1c8a231e2391

Quote from NEWS:
The RPC implementation in libc is obsoleted. Old programs keep working
but new programs cannot be linked with the routines in libc anymore.
Programs in need of RPC functionality must be linked against TI-RPC.
The TI-RPC implemtation is IPv6 enabled and there are other benefits.
Implemented by Ulrich Drepper.

This was removed in Fedora 28, Removal of Sun RPC Interfaces From glibc:
https://fedoraproject.org/wiki/Changes/SunRPCRemoval

This fixes issue #392

Signed-off-by: Alex Wigen