As with recursive make, BUILT_SOURCES is harmful because of inadequate
dependency information. We can achieve a similar effect with by using
order-only dependencies: These force the headers to be up-to-date
before anything that might require them is built: but rebuilds are only
triggered based on the accurate dependency information generated by the
normal mechanisms.
Unfortunately, order-only dependencies are a GNU make feature, so we use
a hack which should fall back to ordinary dependencies on other make
implementations. The worst effect of using ordinary dependencies will
be that files might be needlessly rebuilt when a header changes.