This converts the CMake license to a pure 3-clause OSI-approved BSD
License. We drop the previous license clause requiring modified
versions to be plainly marked. We also update the CMake copyright to
cover the full development time range.
The commit "Remove barely-used cmMakefile::AddCacheDefinition" broke
option() calls that pass a non-boolean default value. We restore the
old behavior by always coercing the value to 'ON' or 'OFF'.
The set(CACHE) and option() commands should always expose the cache
value. Previously we failed to expose the value when it was already set
if a local variable definition hid it. When set to NEW, this policy
tells the commands to always remove the local variable definition to
expose the cache value. See issue #9008.
The boolean overload of this method was used only to implement option().
We re-implement option() in terms of the main method and removes the
now-unused signature. This removes some duplicate code that had already
fallen behind on changes (it was not removing the local definition
instead of setting it).