In CMake 2.6.3 and below we silently accepted duplicate build
directories whose build files would then conflict.  At first this was
considured purely a bug that confused beginners but would not be used in
a real project.  In CMake 2.6.4 we explicitly made it an error.

However, some real projects took advantage of this as a "feature" and
got lucky that the subtle build errors it can cause did not occur.
Therefore we need a policy to deal with the case more gracefully.
See issue #9173.