clock_gettime is avalible since macOS 10.12 and iOS 10.0. Because of
weak linking, clock_gettime can be build without error with new
macOS/iOS sdk, but the symbol may not exist on the target system.
Explicitly checking the symbol is required.
https://developer.apple.com/library/content/documentation/MacOSX/Conceptual/BPFrameworks/Concepts/WeakLinking.html



Signed-off-by: Wang Bin <wbsecg1@gmail.com>
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>