Variable for disabling find_package() calls.

Every non-REQUIRED find_package() call in a project can be disabled by setting the variable CMAKE_DISABLE_FIND_PACKAGE_<PackageName> to TRUE. This can be used to build a project without an optional package, although that package is installed.

This switch should be used during the initial CMake run. Otherwise if the package has already been found in a previous CMake run, the variables which have been stored in the cache will still be there. In that case it is recommended to remove the cache variables for this package from the cache using the cache editor or cmake(1) -U