Thanks for this. Seeing a warning message for an optional program bothered me, but addressing it was out of my depth.
What do you think about using "false" rather than "no" if the program is not found? That would be more defensive because if "$(TIDY) ..." is executed as "false ...", we can be pretty confident that it will fail consistently, but some developer might have a program called "no" in the path that would do something unexpected. https://codereview.appspot.com/573150043/
