Daniele E. Domenichelli authored Oct 28, 2014 and Brad King committed Oct 31, 2014

This argument allows to set default arguments that are written in the
initial cache file, but that are not forced, and therefore allows the
user to change these values later (CMAKE_ARGS and CMAKE_CACHE_ARGS
always overwrite the values).

Also add some documentation to explain the differences between these 3
arguments.

Extend ExternalProjectLocal test to cover CMAKE_CACHE_DEFAULT_ARGS.