Link error with Poco when using libc++

[rotu]

Ubuntu Focal / Cmake 3.16.3 / Clang 11 / Foxy Master

--- stderr: class_loader                                                                   
ld.lld: error: ../../libclass_loader.so: undefined reference to Poco::SharedLibrary::SharedLibrary(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [test/fviz_case_study/CMakeFiles/class_loader_Test_Fviz_Main.dir/build.make:90: test/fviz_case_study/class_loader_Test_Fviz_Main] Error 1
make[1]: *** [CMakeFiles/Makefile2:403: test/fviz_case_study/CMakeFiles/class_loader_Test_Fviz_Main.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
make: *** [Makefile:141: all] Error 2
---
Failed   <<< class_loader	[ Exited with code 2 ]

[rotu]

I seem to be able to reproduce this with clang versions 9,10,11 (didn't test earlier).
But only when clang has CXXFLAGS+=" -stdlib=libc++".