patch 8.0.0429: options test does not always test everything

Problem:    Options test does not always test everything.
Solution:   Fix dependency for opt_test.vim.  Give a message when opt_test.vim
            was not found.
diff --git a/src/testdir/Makefile b/src/testdir/Makefile
index e44bd5c..d464d67 100644
--- a/src/testdir/Makefile
+++ b/src/testdir/Makefile
@@ -142,5 +142,4 @@
 	@rm vimcmd
 
 opt_test.vim: ../option.c gen_opt_test.vim
-	$(VIMPROG) -u gen_opt_test.vim --noplugin --not-a-term
-
+	$(VIMPROG) -u gen_opt_test.vim --noplugin --not-a-term ../option.c