Recompilation, hs-boot files, -dynamic-too seems broken

The latest in the saga of things I broke.

It seems that if you are using -dynamic-too and hs-boot files (see Agda), then compilation creates .o-boot but not a .dyn_o-boot file. This then confuses the recompilation checking because it looks for the dyn_o file and sees it doesn't exist so it decides to recompile.

I suspect it's probably better to make sure dyn_o-boot gets created as well as this might confuse other build systems.

You can reproduce this by compiling Agda.