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. [1] However, that new meta-component triggers a warning in FindBoost.cmake when 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 [1] and [2] .)
[1] https://github.com/boostorg/boost_install/pull/19
[2] https://github.com/boostorg/boost_install/issues/16
Backport: release
Topic-rename: FindBoost-meta-component-ALL