Build failure should result in the deletion of unbuilt targets

[ihh]

The current behavior (which leaves target files in place even if the build fails while trying to run the recipe for that target) was originally by design, but it is different from GNU make's behavior, and has become enough of a pain in the neck (when crafting & debugging Makefiles) that I'm labeling this as a bug.

Pretty easy to fix I think: just delete the file in the build(T,SL,Opts) rule that prints the FAILED message, in biomake.pl.

May break some tests (and should get its own test either way).