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/maketab.c b/maketab.c
index bb8e317..dbe3d24 100644
--- a/maketab.c
+++ b/maketab.c
@@ -125,8 +125,12 @@
 	for (i = SIZE; --i >= 0; )
 		names[i] = "";
 
-	if ((fp = fopen("ytab.h", "r")) == NULL) {
-		fprintf(stderr, "maketab can't open ytab.h!\n");
+	if (argc != 2) {
+		fprintf(stderr, "usage: maketab YTAB_H\n");
+		exit(1);
+	}
+	if ((fp = fopen(argv[1], "r")) == NULL) {
+		fprintf(stderr, "maketab can't open %s!\n", argv[1]);
 		exit(1);
 	}
 	printf("static char *printname[%d] = {\n", SIZE);