CMake: Try to (un-)hide private library symbols

Detect support for compiler symbol visibility flags and apply those
according to CURL_HIDDEN_SYMBOLS option.
It should work true to the autotools build except it tries to unhide
symbols on Windows when requested and prints warning if it fails.

Ref: https://github.com/curl/curl/issues/981#issuecomment-242665951
Reported-by: Daniel Stenberg