Recent glibc-2.26 no longer provides support for SunRPC, and libsigrok's
build support failed to detect the presence of libtirpc in that case.
https://wiki.gentoo.org/wiki/Glibc_2.26_porting_notes/RPC_implementation
Extend the check, use either SunRPC or TI-RPC. Which re-enables VXI on
recent setups. This resolves bug #1473.