[Cmake] Supressing errors for missing subdirectories

[Amitha Perera]

I'd like to implement a feature where error messages for missing
subdirectories can be toggled on and off. This would allow me, in the
vxl context, to remove libraries in which I'm not interested and not
have to modify parent CMakeLists.txt files.

The easiest way to do this is to have a cached (or otherwise) boolean
variable. Two questions:
- Does anyone object to this, or have a better way of doing it?
- How should such a variable be documented? That is, should be be a
  "permanent" variable, or should it be created by a project's
  CMakeLists.txt?

Amitha.