Cmake fixes (#135)
Cmake fixes (#135)

* CMake: fix empty message when BUILD_TESTING is OFF

At this time, cctz has dependencies only when BUILD_TESTING is ON.
This means that the feature summary has nothing to report
when BUILD_TESTING is OFF.

Before this change, CMake would produce an empty status line:

    $ cmake -DBUILD_TESTING=OFF .
    --
    -- Configuring done
    -- Generating done
    -- Build files have been written to: .../github.com/google/cctz/build

CMake 3.8, introduced a QUIET_ON_EMPTY flag,
which avoid printing this empty status.
We use enable this flag on CMake >= 3.8.

* CMake: fix Clang compile feature detection on macOS (#134)
google/cctz,google/cctz