If your generated files are considered outputs that live elsewhere, you
need a way to tell maketab where ytab.h actually is.
Specifically, I'm trying to avoid checking in generated files in
Android's AOSP tree's copy of one-true-awk.
The old ytab.o rule produced ytab.c and ytab.h too, but this wasn't
visible as dependencies, so make -j would fail. Make this explicit, and
use the implicit .c.o rule rather than writing out the $(CC) command.
