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 <[email protected]>
Signed-off-by: Ben Pfaff <[email protected]>

Author: Richard Oliver

AUTHORS.rst

@@ -263,6 +263,7 @@ Raymond Burkholder              [email protected]
 Reid Price                      [email protected]
 Remko Tronçon                   [email protected]
 Rich Lane                       [email protected]
+Richard Oliver                  [email protected]
 Rishi Bamba                     [email protected]
 Rob Adams                       [email protected]
 Robert Åkerblom-Andersson       [email protected]

configure.ac

@@ -112,7 +112,7 @@ AC_CHECK_MEMBERS([struct mmsghdr.msg_len], [], [], [[#include <sys/socket.h>]])
 AC_CHECK_MEMBERS([struct sockaddr_in6.sin6_scope_id], [], [],
   [[#include <sys/socket.h>
 #include <netinet/in.h>]])
-AC_CHECK_FUNCS([mlockall strnlen getloadavg statvfs getmntent_r sendmmsg])
+AC_CHECK_FUNCS([mlockall strnlen getloadavg statvfs getmntent_r sendmmsg clock_gettime])
 AC_CHECK_HEADERS([mntent.h sys/statvfs.h linux/types.h linux/if_ether.h stdatomic.h])
 AC_CHECK_HEADERS([net/if_mib.h], [], [], [[#include <sys/types.h>
 #include <net/if.h>]])

lib/timeval.c

@@ -41,7 +41,7 @@
 
 VLOG_DEFINE_THIS_MODULE(timeval);
 
-#if defined(_WIN32) || defined(__MACH__)
+#if !defined(HAVE_CLOCK_GETTIME)
 typedef unsigned int clockid_t;
 static int clock_gettime(clock_t id, struct timespec *ts);
 
@@ -52,7 +52,7 @@ static int clock_gettime(clock_t id, struct timespec *ts);
 #ifndef CLOCK_REALTIME
 #define CLOCK_REALTIME 2
 #endif
-#endif /* defined(_WIN32) || defined(__MACH__) */
+#endif /* !defined(HAVE_CLOCK_GETTIME) */
 
 #ifdef _WIN32
 /* Number of 100 ns intervals from January 1, 1601 till January 1, 1970. */
@@ -453,7 +453,7 @@ clock_gettime(clock_t id, struct timespec *ts)
 }
 #endif /* _WIN32 */
 
-#ifdef __MACH__
+#if defined(__MACH__) && !defined(HAVE_CLOCK_GETTIME)
 #include <mach/clock.h>
 #include <mach/mach.h>
 static int