sanity: autoconf check for sys/select.h
sanity: add libc/stdlib sanity checks

These are meant to test our back-compat stubs, but they are enabled for all
builds for the sake of consistency.
sanity: hook up sanity checks
Merge pull request #4339

92a6220 sanity: hook up sanity checks (Cory Fields)
679240d sanity: add libc/stdlib sanity checks (Cory Fields)
11404af sanity: autoconf check for sys/select.h (Cory Fields)