build: do not suppress errors computing target signature

I forgot to check an error return. Oops. Too bad nobody has invented a
language construct that would make my program crash when I screw up,
rather than silently suppressing errors when I screw up. Ah well,
that's just crazy dreaming...
build script: initial crude attempt at incremental testing

Doing this right requires a lot more work, starting with plugins to
dynamically discover the list of fubsy/* packages and loop over them.
This is a useful first attempt, though, and it's good enough for a
small project with a fast compiler and fast tests.
build script: use build tags (I defined the variable but did not use it!)