Add workaround for finding rpc includes on CentOS8.
On CentOS8 RPC library comes from tirpc package and headers are installed in non-standard location. This patch adds logic to cmake to locate the headers and new library. One complication is that LCG has its own copy of pkg-config which is broken, need to make sure that it's not used.
Tested on CentOS7 and CentOS8, needs more testing on MacOS/Ubuntu.