which(1) accepts both relative/absolute paths as well as lone binary
names. Set KYUA to kyua and use which(1) to confirm that it can find one;
if it cannot, just advise the user to set KYUA directly to the kyua binary
rather than assuming a relative location from LOCALBASE.
This allows make check to be operated with the version of kyua in base
without losing the flexibility of specifying another one.