[CMake] out of source command line flag

[Brad King]

Alan W. Irwin wrote:
> The fact remains some projects would like to drop the
> <path-to-existing-build> interpretation altogether (for the reasons I
> mentioned) and other projects will want to preserve it (for the reasons you
> mentioned).

The path-to-existing-build interpretation is not the cause of the problems
with trying out-of-source builds after trying an in-source build.  The problem
is that files generated in the source tree by an in-source build attempt,
such as generated header files, can break out-of-source builds.  I have
another solution in mind which I'll post elsewhere in this thread shortly.

> Can't the needs of both groups be served by a policy?

Policies are only meant for fixing behavior while maintaining compatibility.
They are not meant for switching between two supported behaviors.  The OLD
behavior is always unsupported (except for compatibility) and the NEW one
is always supported.

-Brad