Spurious -I/usr/include/tirpc is added when autodetecting libtirpc in linker path

[roystgnr]

A Cardinal+OpenMC user ran into this, on a system where libtirpc is necessary for XDR support but where the headers are still installed in /usr/include/rpc/, not /usr/include/tirpc/rpc/. Our first attempt to detect XDR support fails (because we don't yet add -ltirpc and the linker has missing symbols), then our next attempt (where we add -I/usr/include/tirpc to the compiler flags and -ltirpc to the linker's) succeeds (because we aren't giving the compiler anything like -Wmissing-include-dirs -Werror), so our m4 concludes that those flags are needed and adds them both. Our libmesh-config script and our pkgconfig files then have that flag, and anything that screams about invalid include directories (like the compiler options above, or reportedly the cmake configuration for Cardinal with OpenMC, which dies with Imported target "PkgConfig::LIBMESH" includes non-existent path "/usr/include/tirpc") will fail.

This isn't urgent, since there's a workaround: --with-xdr-include=/usr/include goes straight to the "try to link with -ltirpc" configure path and succeeds without adding any invalid include directories. But this was annoying to debug so I figured I'd at least get the error message and the workaround googleable right away.