FindBoost: Allow testing for multiple compiler suffixes
This is to permit VS2017 to make use of the -vc141
suffix or fall back to the compatible -vc140
suffix to use Boost libraries built with VS2015.
Boost_COMPILER
or the internal _boost_COMPILER
can now contain a list of compilers to check for. For VS2017 we use this list: -vc141;-vc140
to support use of Boost built against VS2017 or VS2015, preferring VS2107.
Topic-rename: FindBoost-vs-toolset-fallback
Fixes: #16631 (closed)