cmake: Remove force from IssueMessage API

The force parameter is ugly and makes the method harder to reason about
(issues the message ... but maybe it doesn't ... but then again you can
force it).  It is a violation of

 https://en.wikipedia.org/wiki/Interface_segregation_principle

and is the kind of thing described in a recent blog here:

 http://code.joejag.com/2016/anti-if-the-missing-patterns.html

 "Any time you see this you actually have two methods bundled into one.
  That boolean represents an opportunity to name a concept in your code."