openpbs-2.3.16-r2 wants the corresponding patch file files/openpbs-2.3.16-r2-errno-fixup.patch, but this file does not exist. While the -r1 version builds and installs fine, it does not PROVIDE="virtual/pbs". Combining this with Bug 101326 leads to the conclusion that there is no buildable package to provide virtual/pbs, and so a build of sys-cluster/lam-mpi-7.1.1-r2 is guaranteed to fail if one should happen to USE=pbs. (Yes, openpbs is limited distribution. But I registered for it, and when instructions are followed, openpbs-2.3.16-r1 builds and installs as expected.)
Fixed.