CMake: Don't build unit tests if private symbols are hidden

This only excludes building unit tests from default build ( 'all' Make
target or "Build Solution" in VisualStudio). The projects and Make
targets will still be generated and shown in supporting IDEs.

Fixes https://github.com/curl/curl/issues/981
Reported-by: Randy Armstrong

Closes https://github.com/curl/curl/pull/990