Avoid GNU make re-exec when adding dependencies to Makefile

GNU make will re-exec if (it thinks that) the Makefile has changed.
Just having the target Makefile seems to make it think it has, so we
end up in a look where GNU make re-execs for ever.

The fix is easy, just remove the Makefile target and have the depend
target run the recipe on its own instead of depending on Makefile.

Reviewed-by: Rich Salz <rsalz@openssl.org>

Configurations/unix-Makefile.tmpl

@@ -249,8 +249,7 @@ clean: libclean
 	rm -f $(TARFILE)
 
 # This exists solely for those who still type 'make depend'
-depend: Makefile
-Makefile: FORCE
+depend:
 	@( sed -e '/^# DO NOT DELETE THIS LINE.*/,$$d' < Makefile; \
 	  echo '# DO NOT DELETE THIS LINE -- make depend depends on it.'; \
 	  echo; \