Install CMake find_package support files

Configure and install CMake config mode support files for the
find_package command. Install versioning file so that clients can
specify a minimum or exact version. Install main vdt-config file
to set include and library paths, the latter via loading an export file
created automatically by CMake.

Set a variable vdt_IS_PRELOADABLE in the config file to indicate if the
vdt library referenced supplies preload signatures.

Clients of vdt using CMake can simply use find_package(vdt) to use it.