Create CMake Policy CMP0015 to fix set(CACHE)

Create CMake Policy CMP0015 to fix set(CACHE)

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.