cmake: Issue message independent of cmMakefile definition

The makefile is only used when called by the cmMessageCommand, so inline
the use of it there.  It otherwise creates an undesirable dependency on
cmMakefile for issuing messages in the cmake instance, a violation of
the Interface Segregation Principle.

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

This also makes it more explicit that the variable definitions only
affect the message() command.  If an AUTHOR_WARNING is issued for any
other reason, it is not affected.  To affect that, it is necessary to
set the cache variable instead of the regular variable.

This is an unfortunate interface quirk, but one which can't be fixed
easily now.