This adds copyright/license notification blocks CMake's non-find
modules. Most of the modules had no notices at all. Some had notices
referring to the BSD license already. This commit normalizes existing
notices and adds missing notices.
- Policy is WARN by default so projects will build
as they did in 2.4 without user intervention
- Remove CMAKE_LINK_OLD_PATHS variable since it was
never in a release and the policy supercedes it
- Report target creation backtrace in warning message
since policy should be set by that point
