I personally would view building something for X client a viable option if it was going to be a large sum of money. Then use that money to fund the company and the product you really want to build and sell.

It can be hard to get that one or two clients who can fund the company, but it is a viable strategy as long as you have the right mindset. We, Datalanche, had this opportunity but unfortunately (like most deals) it didn't work out. Had we successfully made the deal, it would have been a multi-million dollar contract which would have easily funded the entire company and the flagship product and we wouldn't have given away any control.