timeval: Check for OS-provided clock_gettime on macOS

timeval: Check for OS-provided clock_gettime on macOS

[Problem]
Compilation error on newer versions of macOS (Sierra onwards) due to
multiple declarations of clock_gettime.

[Solution]
Have configure check for clock_gettime and check this result in
timeval to avoid incorrectly declaring/defining clock_gettime again.

[Testing]
Source code now successfully builds on macOS.

Signed-off-by: Richard Oliver <richard@richard-oliver.co.uk>
Signed-off-by: Ben Pfaff <blp@ovn.org>