[boost-1.70.0] Linker error with llvm toolset in VS2017

My setup: I use static boost-1.70.0 libraries with PlatformToolset v141 (Visual Studio 2017) and
address-model=64 architecture=x86 runtime-link=shared on Windows 7.

When trying to link with PlatformToolset LLVM (clang-cl and lld-link based on clang-8.0), I get a linker error due to undefined symbols because BOOST_THREAD_DECL is resolved to __declspec(dllimport) even though I link statically. The rest of boost is working fine ... if you have doubts about my setup ;-)

The good news is that I already found a workaround: define BOOST_THREAD_USE_LIB globally whenever I use clang-cl.
This solves the issue that clang-cl seems to be not detected correctly in lines 471 to 477 of boost/thread/detail/config.hpp