using clang-cl with 1.74-rc2 forces auto-link to search for clangw10 libs instead of using existing microsoft-cl built libs

From: Dennis Luehring (dl.soluz_at_[hidden])
Date: 2020-08-12 13:58:58

using latest VS2010 and llvm/clang 10.0.0 release (10.0.1 is a litte bit
more recent but no windows builds available)

compiler set to 2017 microsoft-cl in VS2017:
libboost_serialization-vc141-mt-gd-x32-1_74.lib

compiler set to clang-cl in VS2017:
libboost_serialization-clangw10-mt-gd-x32-1_74.lib

that behavior is new to 1.74 - older boosts versions 1.69..1.73 would
use the vc141 libs
what makes sense because clang-cl is the microsoft-cl compatible replacement

normaly there is no need to switch any external libs when using clang-cl
instead of microsoft-cl