CMakeLists.txt for asio should specify dependency to Threads

[lordjaxom]

When importing Boost::asio from an install tree with find_package(Boost CONFIG COMPONENTS asio) and linking with target_link_libraries(myexe Boost::asio), the pthread library should be imported automatically.

See attached (proposed) patch.
boost-1.81.0-z1-asio-threads.patch

Sorry if this is the wrong place for the issue (since the repo doesn't contain CMakeLists.txt), but I can't file an issue in boostorg/asio :-(

[lordjaxom]

Since CMake-support is not part of standalone asio, and boostorg/asio doesn't permit issues but pull-requests, but doesn't seem to be frequented much, I've added a pull-request there and hereby link it here :-)
boostorg/asio#413