CURL::libcurl INTERFACE_LINK_LIBRARIES uses -lpthread instead of Threads::Threads

[reddwarf69]

Not sure why https://github.com/curl/curl/blob/master/CMakeLists.txt#L289 uses CMAKE_THREAD_LIBS_INIT when https://github.com/curl/curl/blob/master/CMakeLists.txt#L394 uses OpenSSL::SSL OpenSSL::Crypto.

At the very least this stop the user from getting the THREADS_PREFER_PTHREAD_FLAG usage honoured.

[reddwarf69]

FWIW I am personally using https://github.com/RedDwarf69/curl/tree/cmake. I'm not creating PRs because I have not tried to make it ready for that (e.g I'm increasing the minimum required cmake version... and I have no idea how much you want to keep cmake 3.2 support).
I'm solving this with reddwarf69@d1f0314

[snikulov]

@reddwarf69 you changes (at least use Threads::Threads) does not require the latest CMake. Imported target Threads::Threads already present since v3.1