maketab: support build systems with read-only source.

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.
diff --git a/makefile b/makefile
index 3f3c3c2..a4f1b8a 100644
--- a/makefile
+++ b/makefile
@@ -67,7 +67,7 @@
 ytab.h:	ytab.c
 
 proctab.c:	maketab
-	./maketab >proctab.c
+	./maketab ytab.h >proctab.c
 
 maketab:	ytab.h maketab.c
 	$(CC) $(CFLAGS) maketab.c -o maketab