FindBoost: Prevent warning due to new meta-component "ALL" of Boost 1.73
Boost 1.73 will introduce a new meta-component
ALL in its
BoostConfig.cmake script, which automatically searches and creates targets for all (installed) Boost-libraries.  However, that new meta-component triggers a warning in
BoostConfig.cmake is triggered through it.
The current merge-request prevents this warning.
Note: The purpose of this new
ALL meta-component is to release the developer of the burden to know in advance which Boost libraries will be required. (For more detailed background discussion see  and  .)