cmake: Threads detection update. ref: #1702

[snikulov]

Some improvement for #1702.

• make ENABLE_ARES and ENABLE_THREADED_RESOLVER (by default ENABLE_ARES will win if enabled)
• enforce Threads package if ENABLE_THREADED_RESOLVER set to ON.
• Variables will be set to ON according to CMake findings

[mention-bot]

@snikulov, thanks for your PR! By analyzing the history of the files in this pull request, we identified @Sukender, @billhoffman and @ligfx to be potential reviewers.

[coveralls]

Coverage remained the same at 75.267% when pulling 0e93fab on snikulov:cmake_pthread_test into 0139545 on curl:master.

[bagder]

So what will the default resolver be if nothing is set by the user?

[snikulov]

@bagder threaded.

[bagder]

Thanks!